I did a database search of kit airplanes to return the
fastest 2-person aircraft under your price limits. You can probably buy ones
already built less expensively than you can build a new one yourself:
Under $30K: Look for an older Van's RV4, RV7, or RV8. These
will cruise 165 – 180 knots (190 – 207 mph).
Under $50K: A Mustang II will push 190 knots (218 mph!)
For a 4-place kit plane, check out the AeroCanard. It will
go around 180 knots. I don't know how many of them are flying, so it may be
hard to find one for sale, but you can probably pick it up for under $50k.
In certified (not homebuilt) aircraft, Aviation Consumer
just reported that a 1969 Bellanca Viking (model 17-31) sells for around $30K
and cruises at 190 kts (218 mph).
Be careful when buying older airplanes, especially
high-performance ones. In some cases, an engine overhaul will cost $40,000, so
a plane that looks like a bargain could really be a maintenance disaster in