hello gremlin,
building the kit is quite straightforward thing.:...it's all in the manual that will come with the kit. read that first, really do so. building it then, it's not very difficult, since the pcb is already populated and all you have to solder are the pots and cables. of course, it needs a bit concentration and some basic soldering experiences are helpful. there are quite some vids on y.t. about soldering, if you're not too familiar with the iron.......
make yr first solder joints maybe on a cheap electronic kit of some sort, before you get yr hands on a theremin, imnho. check out www.instructables.com also for basic soldering/electronics. or get that book: handmade electronic music a nice work by a nice man.
but to get a bit prepared, you need at least:(beside a phillips screwdriver,a wrench..) a fine-tipped soldering iron. thin,flux-cored solder ( not lead free !). small pliers and cutters. a multimeter. is always good to have for troubleshooting....i think that's the basics. (most certainly this can be expanded to also get a frequency counter, etc...:-)
the tuning of the finished unit is maybe the more difficult thing, the circuit board comes precalibrated, but needs some adjustement regarding the housing etc...just a hint: don't turn L5, unless you exactly know why. (read some posts around here about it.) the manual is a bit unclear about that.
hope that helps.
(it's oh so quite....it's oh so still....