I think for 600rwhp, your $4000 budget is more realistic than than $2000. And you'll need to do as much of the work yourself as you can.
The major parts alone like a heat exchanger, lower balancer and pulley, and cold air intake will take you to right about $2000. Then you have the lower cost items like thermostat, spark plugs, belts, fluids and a tune. Those will nickel and dime you up another $500 to $700 depending on how good of a deal you get on the tune. Throw in headers and high flow cats, now you hit $4000.
Getting 600rwhp doesn't cost that much. It starts to get expensive when you want to get over 700rwhp. And by expensive, I mean you start spending $1000 to $3000 for single components that may get you 30-40hp.
To your point of "old habits", at the end of the day, we're just balancing how much we're willing to spend to improve the efficiency of our old fashioned pushrod air pump!
