I need some specs estimations on a 1994 Z28 I'm looking into buying. It is NOT stock. Very good condition and very well maintained. Owner is asking $4900 bottom dollar. The owner says it puts down about 320hp to the wheels.

Modifications: GM Performance axle with 3:23 gears. Functional ram air hood. Full line MSD ignition. Ported and polished throttle body with slp air foil. Descreened maf sensor. Flowmaster catback with gutted cats. 160 thermostat. March performance underdrive pulley. B&M transgo shiftkit. Xenon body kit. Bmr suspension. Lowered on Eibach prokit springs.

About how much horses should I be putting to the wheels? Is it a good buy?

There's an LT1 dyno archive here: http://www.ls1tech.com/forums/showthread.php?t=303172

My guess would be around 275RWHP. You should be able to get it up in the 325RWHP range with a cam swap.