halaci: 1.) Found different gems during the play. Are these simply to sell, or raw materials for potions?
There's no other use, but to sell them.
halaci: 2.) Connecting to the first question: I haven't used much potion except healing ones on my party (now lvl16 thief). I saw that I can buy everything, except the permanent potions which the herbalist woman makes. Is it necessary to use alchemy in the game later at all? Or the situation will remain the same: have money - have potion?
I don't think the alchemy skill is very important. Once you can use it to make some powerful potions, you also should have enough money to buy them. And to be honest: I don't think I used a single potion except health, stamina and mana during the whole game (maybe strengh once).
halaci: 3.) Scrolls. Are there absolutely random at the vendors (meaning that I'm extremely unlucky), or is there a system in it? I mean at lvl 16 I could buy a lot of different spells, some of them are avove student level (which I think technically impossible to reach at this point), but I haven't found heal/resurrection/purge spells either in dungeons or at vendors, not even one. It's a bit annoying that I have to run back to the town every time a spider bites a party member or reload a number of times just because one of them dies in a more-than-two opponents fight.
If there is a system, console me, what level should I reach to be able to play a smoother gameplay.
Absolutely random. But normally you should have these spells by now. If you really want them, go to a merchant (or better to a priest) and if he doesn't have them leave and come back right away. After a few tries they should be there.