I play a technological char at the moment. Right now I'm not exactly sure which disciplines I should chose. I have 2 levels in electricy and 3 levels in smithy. So now the questions are:

1) Is it worth to learn all schematics in those two disciplines, e.g. for the tesla gun or the armor in smithy.
2) I consider also investing in gun smithy. Is it worth it? Somewhere I read that you can buy or find those weapons during the game. Is that true? Also the elephant gun is needed for the tesla gun.
3) Is it possible to master all three disciplines with the level cap remover?

Thanks!

Depends on what you want your character to be like. If you go full Smithy to get the heaviest armor you don't need the armor in the Electric discipline, or the shocking staff. But if you are ultimately trying to get a Tesla Gun then you need full Electricity and either full (or pretty close) Gunsmithy as well.

Are you a gun using character? If not/don't want to worry about the Tesla Gun then don't go higher than the charged rings (because they are the most important part of the Electricity discipline). If you want to use guns but don't think you'll have the points (/want them in other things) just put points in Gunsmithy to be able to eventually (near the end of the game) make the best one handed gun in the game. The other guns, yes, you can find in the game world.

In terms of mastering all three disiplines: yes, it is possible. But you might be wasting points depending on what you are doing/how else you have spent points. There aren't areas that respawn enemies (technically there is one, but it is only really usefully for a lowish level character, otherwise it takes ages and ages to level there), so to actually get a level 50+ character early enough in the main quest you have to do a lot of side quests and exploring/know the game fairly well. So yes, you can do it, but it might be better to not plan for it.

1) The answer to this depends on your playing style. Almost any combination of disciplines can be worth it, if for no other reason than to see for yourself how useful they are.

2) It's really only worth it if you're planning to master Firearms as well. Guns have a number of drawbacks:

a) Bullets are a pain in the neck to find (or make) enough of.
b) As a fighting style, Firearms isn't very good until you invest 4 or 5 points in it.
c) The Firearms skill requires points in Perception (PE), which otherwise you wouldn't need to bother with.

And yes, there is at least one elephant gun in the game, depending on how you handle an optional quest you can receive in the city of the elves. I can send you spoilers by PM, if you want.

3) With the level cap remover, it's not a problem. Even without it, it's doable; but you might have to compromise elsewhere, such as not putting points in Strength (not needed with guns anyway) or not taking on Persuasion-based quests.

Thanks for your tips.
I plan on playing with firearms, so I think will invest my points into electricity. If the other guns can be found during the game I won't invest points in gun smithy at the moment.
As for smithy the goggled helmet would be nice. But I think maybe Magnus can build the helm of vision and then I can buy some manuals to do the rest.

Compared to normal guns..the Teslagun has one big bonus, the damagetyp.
Several enemys are resistant to normal physical damage but, as far as i remember, none that is resistant against electrical damage.

If you're planning on Firearms as your main fighting skill, then I would definitely recommend going for the Tesla Gun. It's easily your best bet in the long run.

Definitely pay attention to the tech skills of your companions. Some of them (as you mentioned) are or will be capable of creating items you may want or need.

Some of the best tech items you would want to craft:

Chapeau of Magnetic Inversion
Tesla Gun

Droch's Warbringer (endgame)
Elephant Gun
Tranquilizer Gun

Stun Grenades
Explosive Grenades
Dynamite
Pyrotechnic Axe

Balanced Sword
Electro Armor
Elite Plate
Machined Plate

Guide to Arcanum Schematics
http://www.rpgcodex.net/forums/index.php?threads/guide-to-arcanum-schematics-complete.54412/

FAQ for new players
http://www.gamefaqs.com/pc/914155-arcanum-of-steamworks-and-magick-obscura/faqs/63974

Best of luck mate

Flow Disruptor may be worthwhile too - especially if you're planning on using a one-handed gun or tech melee weapon

One of my recent characters had a Chapeau of Magnetic Inversion and a Flow Disruptor shield and I don't think either used any charges (maybe the Unofficial Arcanum Patch changed them for some reason?)

AFAIR Chapeau of Magnetic Inversion does use charges; Flow Disruptor doesn't (with UAP, I can't recall if it's true for unpatched, vanilla game).