No, not even when using the plus version of the Unofficial Patch for several reasons: 1) dialogue options are out because there are no voice-overs that could be used, 2) stealth only is unrealistic in the WoD because the endgame bosses all have discplines or powers to see through that and 3) I think that in the setup of Bloodlines there is just no way for all main factions to just let you live. But then by choosing certain sides, you can at least skip one of the endgame fighting sections...

You can get away with a weak fighter in the early parts of the game, but your character will have to fight by the middle and end.

Players who try non-combat characters eventually run into fights they cannot win.

Have to agree with the posters above me - the game is unfortunately not set up for pure non-combat since you have to fight at least one of the end-game bosses, and they're pretty challenging.

That said, I don't think I've ever had my fighting skills maxed out by the end, so you don't need to worry THAT much - plus you've got blood buff plus potentially other skills to help you to block/avoid damage or temporarily increase some combat-related skills. But I'd definitely ensure you have at least ranged combat skills (for the bosses), and ideally some decent melee skills as well (since IMO melee is often better for most other fights, though I suspect this could be personal preference).

How viable would it be to add some kind powerful weapon where the requirements to properly use it is stealth and persuasion, instead of the usual weapon requirements? If it was possible, it might be a great workaround for those kind of characters.

Well, if you're willing to go that far I'd say just cheat and give your character a bunch of extra skill points instead. The game really isn't about the fighting anyway. You could even wait until you hit a particularly difficult battle.

I've played Bloodlines many times, and I don't consider the combat requirements a problem. It's only difficult for some first-time players who don't understand that they'll eventually have to fight.

The game informs you that you'll need to fight eventually, but sometimes people ignore that, and because you don't get punished for ignoring it until later on, it can be a trap.

You will need some combat prowess to battle some of the tougher bosses.
That doesn't mean you are not able to go full persuasion/stealth otherwise.

if you do not want to use combat skills at all, then ok, that's a bit hard to do in the game.
If you however just want to max out in your preferred area and still be able to battle the bosses reasonably enough, then that is perfectly duable.

What clan(s) where you looking at, that were interesting you the most; just to get an idea, of how we could help you achieve your character goal?

I think this is pretty much impossible as most of the weapon system is completely hard-coded. But there always is the flamethrower! It's a very powerful weapon working well regardless of any combat stats, so if you missed out on those, just buy it at the earliest opportunity from Mercurio and keep the fuel stacked before each boss encounter. Be aware though that fast computers make it eat up fuel very fast, so only use short controlled bursts.

I'm torn between Toreador, Tremere and Ventrue.
Tending to Tremere for the blood magic aspect and blood steal. (always enjoyed the necromancer classes especially with blood magic, case in point my main character in Guild Wars is a blood magic Necro)

I'm a horrible min/maxer when it comes to gaming, i'm currently playing a character just like that. It starts you off with a solid focus in some of the things you want to specialize in, so that's why i thought i'd chime in when i saw your question. I saw you posting this question on reddit as well and the discussion is now going for a few days, which leads me to believe a) you may be overthinking this, b) you might be someone who is open to a bit of character optimization to get what the want.

Your preferred clans are good options to achieve what you want.
I don't want to go too meta gamey just yet, but know that there are ways to go about building your character, that essentially allow you to become close to a jack of all trades later in the game.
You may be able to spend most your points on the things you prefer, while the other skills come to you by other means.
You'd be a bit limited about when to raise what for a good part of the game, but afterwards you'd be able to fully focus on your desired path, while being equipped enough in other areas, should the game limit your options to progress further using your area of expertise.

Tremere, Ventrue and Toreador are well rounded clans that are a good start in the game and there is plenty of replay value with other clans, where you could ignore min/maxing in a later playthrough.

Let me know if this is something that might interest you, or if you'd rather go about your playing with untainted RP morals :D

Tremere is fun. It should make for an interesting first play, if you choose it.

If you focus on blood magic you should not have too much trouble with combat, even against bosses. You'll need at least passable skill with some kind of backup weapon though, so you don't use up your blood pool too quickly.

Tremere is arguably the most powerful clan by the late-game.

I keep hearing that but, having palyed each dozens of times, I find them to be pretty much balanced.

This is true. I think most skills (including combat skills) can be raised with one or two skill books - they each add one to your level in that skill, but only if your skill is below a certain level at the time.

So even if you don't want to spoil yourself overly much, I'd recommend checking out what those skill books are, their requirements and approximate locations. Missing out on a free skill point upgrade sucks, min-maxing or not.

There are also a couple of other places where you get a free skill upgrade - and again, you need to be below a certain level to actually get the benefit.
Tremere does kick ass for a number of reasons as others have said: you can steal blood, their blood purge ability affects multiple foes and even works against the supernatural (somewhat), and if you play the plus patch they can also use their dominate in dialogs (albeit not as many as a Ventrue, but enough to be noticeable).

IMO the best blood magic skill (possibly even more useful than blood purge) is the blood shield - it lasts until its destroyed (which can take a while), and IIRC it reduces incoming damage by a percentage, unlike other defensive skills or armour, which reduce damage by a (small) flat amount.

I've definitely found combat noticeably easier as a Tremere than other classes such as Malkavian or Ventrue (though admittedly none of these are specifcally combat-focused)

Note that in the plus patch it's possible to gain an additional (i.e. fourth) special discipline (dependent on choices at a certain point). Although you're limited to levelling it to only half way, getting fortitude as a Tremere makes you much more resilient, therefore helping combat even more.

From memory, the ranged feat doesn't restrict you from the guns you can use. It increases damage and aim.
Get rapid fire guns and as much ammo as possible. Cash should be a non issue by the end game if you do enough sidequests.
Stock on fresh blood too from Vendal, you will take a lot of damage against bosses.

With enough patience, you should be able to make it.