Welcome!
Building a mech is a fun experience. If you have a kit you're looking at, definitely look at a few soldering tutorials on youtube. maybe find some old crap to try and test your hand at. Better to fry some garbage than to fry your PCB.
Aliexpress has a lot of the components you'll want to use, like case, pcb, screws, and caps, too. Or you can dive right into one of the higher end boards if you're feeling brave!