I'd say have a big budget, like others have said, JEM parts aren't easily availalbe so you may have to caniballise several whole guitars.
I'd also say keep ALL of ALL the guitars you use, the resale value on a bitser is very low so you'll probably want to be able to put everything back the way it was in case you do have to sell one day.
I'll also point out the obvious, check that trems fit bodies (the various bridges have slightlyu different routs but that CAN interfere with assembly/operation) check necks fit pockets (aanj vs block heel) and watch for clashing colours
Start with either a RG550 or RG570. You can pick one up for about $300 if you look around. I found a 2001 rg570 for $279 in almost mint condition in the summer. Its the guitar in the picture. If you buy everything in parts, youll spend twice as much. After you have the guitar, strip the paint and sand the sucker down. Next, cut your monkey grip and lions claw if you want it to look like a jem.
After you have done your woodworking, prime it, paint it, clear coat it and polish. After all thats done, you can add whatever hardware you want. This is what I did and it cost a total of about $500. I have 2 more pickups coming and mine will be done. You can do all this in a week if you want, or like me, Ive been doin things here and there. Heres a picture of what I did. I tried to make a PMC, but the colors came out dark. Im going to actually redo the paint when it warms up outside. Go to projectguitar.com for all the instructions on how to's. The easiest part was doin the woodworking.