I just finished the quest given by Sabastion in the Lower Ward to kill Grosuk and now he can change my profession to a mage which I've been searching for a lot. Now I know this game isn't really about combat even though I love its part anyway but I always prefer to be mages in RPGs, at least have a few in my party if it's a party based game but there's a few concerns I've with this.

First, I seem to lose levels. Currently I'm a level 7 fighter and deal decent damage but I die fast so I gather if I become a level 3 mage (which I do if I receive training by him) I will die even faster and with level 3 I'll only have 3 spells per rest (and with the mods I added, I can't rest anywhere and in a long dungeons that will make it very tough especially since I'm low level so no AoE spells or something long lasting with great benefits).

Second, how does the system work about spell slots because I can see this becoming very frustrating, at least in the lower levels and even overall if the game isn't as long as I think it is.

Third, I gather attributes are important for specialist classes that's why I chose to pick high intelligence (as a future mage) and above average in charisma and wisdom (because I prefer multiple dialogue options in a story based game and wisdom seems to fit the game with the returning memories and such). Anything that one is suppose to know about them? I have all those mods in the mod thread and I picked all installations that were recommended including that one that apparently change some of the attribute system to be more like in Baldur's Gate.

Switching to mage won't make your character more vulnerable, you keep your hitpoints and except some weapons you can equip everything a fighter can and get access to protection spells.

The reason why your character dies so fast is that you chose him to be so wimpy by dumping con. You'll have to raise it to 15 to get your first bonus hitpoints however so consider if it's worth it. Raising con from 9 to 14 will only let you regenerate faster (1 HP every 35 instead of every 60 seconds).

Don't raise int too high, 18 is perfect for dialogue and casting, if you have 16-17 you can just use a +2 int tattoo. The more wisdom you have the better.

Wait too high, you mean so I don't have points left for other attributes?

I understand about constitution but I only gain 1 point per level so I'm a bit confused.

I'm still trying to figure out if switching to mage will be worth it, I'm quite confident that I will have some frustrating time ahead because of the few spell slots but I wll try and see.

Stats can never be too high but if you have already 18 int raising a different stat will be more useful.

You only get full hit points at level up if you reach a new level previously not reached by another class, you have 7 fighter levels already so before you reach level 8 you'll only get 1 hit point and no attribute raise.
If you change to a new class you get the first levels quite fast, while a fighter needs additional 61,000 XP from level 7 to 8 and 125,000 from 8 to 9 a mage needs only 60,000 from 1 to 7 and 75,000 from 7 to 9 for example.

On the first playthrough I changed from fighter to mage at level 7 too, and ended up as a level 25 mage or thereabouts at the end of the game. There is no problem changing really, though I suppose it would be more ideal to change to mage even earlier.

There isn't a great deal of fighting anyway, so having few spell slots isn't a huge issue. Just rest more often if you want to be fully 'loaded'.

Just keep in mind that there are huge rewards in terms of added XP by having high wisdom, so it's something that's worth investing in just for that, even all the way up to 25 (where you get +40% XP).

High constitution is quite nice as well, as you will regain hitpoints much faster.