Z4 M? It's an NA engine so probably not so tunable... but surely a very fun car. 330 HP, 3200 lbs
Also, what about the 370Z?
used Porsche Cayman? 2900 lbs, 245 HP (2007) (Probably not an "S" version with your budget.)
used Boxter S? 2950 lbs, 280 HP (2006) 3000 lbs, 295 HP (2007)
