Craig,
You would probably have a pile of cash left in your pocket with that set up. I agree a twin turbo, intercooled Lexus V8 is probably is the best value for money and all round performer.
If emissions compliance isn't an issue I'd look at an older gen-2 SB Chev with a couple of hairdryers on it. If you were carefull where you found the parts I reckon you could build a 1000hp plus motor for under $12K. I'd probably try to source a lot of the parts second hand from the drag racing, V8 Supercar or NASCAR crowd.
You can pick up ported aluminium heads like Brodix AFR etc for around the $2000 mark. There are heaps of top quality cranks rods and pistons etc available new at good prices. Good second hand bowtie and aftermarket blocks come up regularly (One school of thought says a seasoned block is better than a new one). Some good second hand ball bearing turbos, blow off valve etc shouldn't be to hard to find. The ricer crowd has assured us that there is plenty of this stuff around, just check ebay.
This could be done with a Ford SB as a basis too but there seems to be a lot more S/H Chev stuff arround.
The expensive stuff would be the engine management, intercooler, plumbing and tuning. but it should all be possible for under $12K
It might make more HP but where the Lexus wins is if the motor goes bang it'll be a whole lot more cheaper to fix.
The criteria is "quickest most reliable value for money engine possible" More HP doesn't seem to always add up to quicker. A lighter engine with good HP and drivability (no light switch power delivery) might be quicker on the track than a monster motor that spends all it's time spinning the wheels.
One of the criteria was that the motor had to be legal and I wonder how some of these realy big HP motors would go with emissions and noise tests? (not talking about anybody in particular here Craig
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So how about a naturally aspirated LS1 with some head work, cam rods etc. It will reliably make 500HP+ with a linear delivery. It could all be done with the factory ECU (tuned using LS1 Edit) and most of the factory fuel system so no need for an aftermarket ECU. It's feasable that it could meet emissions tests and the engineers like to see a factory ECU too. The engine is a feather weight at 180KG so keeps the total vehicle weight down and the mass centralised. A bar/plate intercooler is jolly heavy and all the plumbing and turbos add to the weight as well. Plus a lot of this weight is hanging out the front away from the center of mass.
Cheers