I just got my 88 Zilla last week but I have been researching them for months. From what I have gathered the 87 models are fine if left stock. Both the 1987 250 & 500 Quadracers have larger reed cages than later models which slightly increases their performance. But when you increase the compression, one of the weak head mounting holes is prone to fracture. I met a guy yesterday with a nice, near original 87 Zilla. He claimed he had owned several before and modded them extensively but is now back to an original 87 because it is much more reliable (i.e costs less to repair/maintain and a LOT less time).
I would avoid any heavily modded bikes. Don't listen when people tell you there are no more original bikes left. There are. I just bought one. You have to be patient and look around. A road trip to find an original one is worth it in the long run. It can take thousands of dollars and weeks or months to fix a basket case. Mine only has an FMF exhaust, reeds, and the original Mikuni rejetted and I rocketed to the top of every dune at Amargosa Valley yesterday (i.e. 200-250 foot dunes at about 45 degrees inclination. I actually could have jumped over the top of the biggest dunes very easily. My 89 250 Quadracer pales in comparison. It is quick, fast, and light, but it struggled to get to the top of the same dunes. I'm definitely in love with my Zilla. I don't even want to screw with the engine. I'd rather lighten the bike if possible rather than make it any more powerful.
I would review cycletrader.com daily to see what is in your area as well as nationally. That website will even notify you by e-mail when the bike(s) you specify are added to their ads.
Good luck