Assuming we're looking at engine's intended for OEM RWD vehicles only, then what tick's most boxes when taking into account (combined):

Price
Ease of engine management
Weight
Performance

Some engines for consideration:

BMW N52B30 (3.0 si)
Honda S2000
Mazda MX5 2.0 (Duratec)
Mazda RX8
Mercedes
Nissan 200SX
Rover V8

Volvo Whiteblock and Volvo Redblock turbo especially, complete donors can be had for peanuts.
Enormous power potential too

V8 makes the best noise. Barry

Duratec has to be approaching the best balance of those criteria. Seeing what Dave (Flak Monkey) has achieved with his superharged install makes it look very appealing.

For out right power, then ready turbo'd motor's from Nissan etc have to be pretty attractive too. The good old Cossie is still (IMHO) a very capable/worthy contender, but costs can be high for big power.

200sx gets my vote 320bhp
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Zetec, I say that because you can pick them up for next to nothing, or, nothing at all...

Bike carbs, Megajolt and your off.. vrooooooooommmm

There's a lot to be said for the Mazda Renesis engine, light weight (for a car engine) and compact, loads of BHP, very high reving and the lack of weight in a 7-esque will overcome the lack of torque

bang for buck i doubt you'll beat a zetec which bolts straight to a type 9 box and comes with the advantage of bolt on parts to keep it cheap

Ford 2.8 or 2.9 V6 (12V or 24V). Takes a type 9 using the capri bellhousing. My cossie 24V cost me £150.

I'd say 3.0 m3 motor, 300+bhp, N/A strong/reliable and sound great.

Oh ITB's as standard too. I'm biased though.

Saab 2.3 turbo mated to omega gearbox will take some beating . Mitsubishi td04 turbo as standard on a lot of models which can be tweaked to 300hp with little fuss . Very strong and smooth engines . The trionic management system is very clever for it's age as the coil pack works as a knock sensor for each cylinder and there Is a software called t5 suite which allows diy mapping and data logging . My road car has a 7" monitor with similar data to the megasquirt . I bought a mint aero last year for £600 which makes 225hp as standard and I've just bought another car with a quaife diff hybrid turbo making over 350hp for £1300 which ic going in my bike engined midi .

im sayin nowt .......

I second the Saab 2.3 300 BHP on standard internals for very little work. HTH Ray

pinto I'll get my coat

I agree with the Volvo camp

Not disputing the Volvo potential just interested to know what's involved in getting to 350 hp compared to the Saab ?

In no order:

Toyota Altezza, Beams 3sge.
Nissan S15(200sx) SR20DET
Honda S2000