Thanks for replies!
groundFungus:
(...) Would a servo or gear motor with encoder work in place of the stepper?
Yes, and no.
It would be nice to have possibility to move things by hand without powering up the motor, but I could live without that.
The main issue for me is that I can't find any good geared DC motor with encoder in reasonable price (the same for the servos). Looking at the gearboxes used in typical constructions I wonder how much rattle would they get after a while...
I've seen some reviews of typical constructions on youtube and I've noticed that there's even a play on shaft in "locked" position on most of them.
Another issue is that even if I could afford good geared motors with good encoders, they're nowhere near the size and weight of a stepper motor. It would probably be more justified if I had something like 30kg of load, but I need max of 5kg, which I think would be easy to handle with a 0.4A Nema 14 stepper + 4:1 reduction based on 48T / 12T pulleys (it would probably be also more stable in "locked" position)
I have to admit that I have no experience in geared motors or servos at all. The above are just my theoretical considerations and conclusions based on what I found on the internet 
so I might be wrong.
jremington:
Motor torque is proportional to current, so you need a certain amount of current to generate the required torque.In any case, steppers are usually the last choice, when battery life is important.
I assume that it's not regardless of size? I've noticed that Nema 17 can be nearly twice as strong as nema 14 with the same voltage, amperage and resistance.
Going back to steppers. I think about one of those:
14HS10-0404S
1.8°
14Ncm(20oz.in)
0.4A (per phase at 12 V)
30ohms
12-24V
14HM11-0404S
0.9°
11Ncm(15.6oz.in)
0.4A (per phase at 10 V)
25ohms
12-24V
17HS08-1004S
1.8 °
13Ncm ( 18,4 oz.in)
1.0 A
3,5 ohms
12-24V