It all boils down to either a screw or a rivet build question as far as the difficulty on the build. Or more like how much it would cost to put one together. Right now I'm making jigs for bending receiver flats which would be the way to go if you're doing a few rifles or just want to say you made the receiver yourself.
Once you have all of the parts then it's down to riveting or using screws. If you end up riveting you'll need to press out the barrel, get either Plinker's rivet jig or modify some bolt cutters to do the trunion rivets, definately need a 12 ton shop press, Max's trigger guard rivet jig plus you'll need a heat source to harden the receiver holes and ejector if you bend your own flats. Don't forget the fun part of drilling or grinding out the old rivets.
If you do a screw build you can possibly get by without taking the barrel out, but you'll still want to check the headspace. Short machine screw drill bits and possibly a center punch to get the rivets drilled out. The better the material of the drill bits you use the better they will cut through the rivets. I haven't seen anybody say you would need anything special as far as the material for the tap so high speed steel should work for those. Make sure to get bottoming taps for doing the front trunion holes.
Another thing to keep in mind is if you build one from a flat the rails have to be welded in place.
It's a whole different ballgame than putting together an AR15. I had made a lower for using 9MM Sten mags from a 0% lower and it almost seemed like less work. Especially since there are so many tools needed to put together an AK to make it look close to a factory build. Once you have all of the tools then you can build one whenever you find a deal on parts kits. That's probably the best part about having the tools laying around.