The best site for almost any info you'll want about a Z32 300ZX is
www.twinturbo.net.
As for engine swaps, they're virtually unheard of in the Z32 community unless you're swapping in a new VG30DETT because your old one blew. Talk of converting NAs and 2+2s to twin turbo is common, but rarely done because of the immense cost. There is supposedly an SR20DET swap in a 300ZX in Japan, but it's pretty blasphemous to put a 4-banger in the engine bay of a born 6-cylinder. Even if it is a lighter engine... hehe.
For fair values on Zs in your area check
www.edmunds.com, www.kbb.com, and
www.nada.com. You'll get a sense for what the cars are worth after looking at some.