You always have to have trade offs in whatever you desire. Some features may be more important than others. Having said that the SR-71 meets most of your requirements.
Cruise around 180-200 kts. (Check, 1,800 knot cruise, oh sorry, you said 180)
Economical - Diesel engine preferably. (MOGAS is also ok) (Check, if you own an oil well)
RG (Check)
2 seats.+ (Check)
Envelope around 6g. for utility cat. (Sorry, quite a bit less)
50-100k $ (Check, they were giving them away for free)