Hey Nathan,
On the eternal quest for 400hp i see.
Im not sure that you could find one for 12-15000 if you dig around for a while. But with that kind of milage on it i would not sugest you go for 400hp without some king of engine work.
Such things like, replace all seals, such as oil pan gasket, hed gasket kit, valve stem seals, and anything else that acts as a sealing surface.
Change all fluids, coolant, brake fluid, oil, clutch fluid, tranny fluid, diff fluid, ect.
Some head work, het the head decked(resurfaced), valve job, while its off get a P&P(port and polish), new valves if the existing ones are burnt, possibly new springs.
Any other wear and tare components. You might discouver that you may need some bottom end work as well. Possibly new thrust bearings, oil squirters, piston rings.
I guess it all really depends on the mechanical condition of the car. If it has been properly maintained then it will be good after some TLC.