I am not sure what question you are trying to ask here. A solid $500 XJ is a pretty good find. Even if you only use the sheet metal, axles, doors, seats and console from it, you could probably part it and make your money back. It sounds like the XJ would work perfect as a donor rig to do the 97+ conversion.
My recommendation for you is to enjoy your truck for the time being, sit back and read through the hundreds/thousands of awesome build threads here. Realize if you go down the 97+ conversion route you will be MJless for quite sometime.
This fellow did the 97+ conversion at a lightning fast pace:
http://comancheclub.com/topic/40605-88-comanche-and-97-xj-about-to-become-one-hello-all/
his is acutally the thread i read on jeepforum.com. No hurry. this makes 3 trucks, 1 car, 1 wrangler, 1 harley for a 3 driver family so no rush but would like it done right. I think i will pick up the XJ if they will come down to 300 and take the axles and the transfer case, maybe the tranny for later use and then do the front clip and seats on the MJ and later on do the motor and dash when a a good motor XJ comes along.