You build airplanes for a hobby, and you're worried about this?!
For the most part you need to be able to cut wood/MDF, and have the resulting cut be square and straight. Having it end up the right size helps, too. It also involves some minor "machining" type of work, like drilling holes with reasonable precision.
I didn't build a JGRO unit, but the basic construction is pretty similar on all of them.
These machines are pretty simple, mechanically...take a look around this site. LOTS of pictures. You'll notice just how simple they really are, when you start really looking.
Look also at the solsylva (sp?) machine...it's inspired. One of my current favorite designs, primarily because of the fully supported rails. Supporting the rails addresses a LOT of the problems with the other designs.
http://solsylva.com/cnc/1aplanshome.html
-- Chuck Knight