k24 with a k20 head. You'll probably need to modify (hack apart) the hood though. I'd say H22, because I have a 5G prelude, but I can tell you that our H motors just don't respond to mods as well as the K motors do. It is probably the most work to put in there, but once you get it in, it'll be the most bang for the buck. (plus the hottest. Not too many people have k series in a rex.)
EDIT: oops, didn't read the "budget" part of your post. For the budget, either build an Integra LS or GSR motor (B18B and B18C motors)