The K24 wouldnt be cheaper than a turbo... but in some ways better. The LS Miata would be crazy but big $$$
I've seen some people swap the F20C for a K24 in S2K's when the F20 blew up... cheaper and more power.
C4S $20K into my 911 it would still be slower and more breaky than said Miata.
I also think a C4 Corvette could be fun, I mean it beat the 911CS in 1989
A 1996 LT4 is an under rated beast