The most cost effective solution would be to rebuild your present engine with some minor performance mods. It sounds like you don't plan on doing anything wild with, but just want a good strong pulling engine that will last a long time. Personally, I would get high quality rings, pinstons, cam, intake manifold, and headers. Also, I would have some head work done. Having the heads ported and polished will show the most power gains and the other add ons will just add to it. A good low to mid RPM cam and intake combo is the Edelbrock performer series. Try a local engine rebuilding shop for the head work. You can save a good deal of money if you do all the disassembly and assembly yourself. It's not very difficult, but does take a few tools not everybody has. There are plenty of good reference books that detail the process. I would bump up the compression ratio slightly. This will help the power, but if you do it right, you can still use low octane fuel. All these modifications basically make your engine "breath" better. The more efficient the air enters and exits your engine, the more power you make and the better mileage you get. (If you keep your foot out of it)
Oh yeah, if you don't have much time to work on it, or a good place to do the project, I would go with a GM performance crate engine. The stock crate engines are just that, stock. One of the big advantages is the warranty you get with the crate engine. If you didn't already have a good engine to rebuild, I would definately got with the GM performance crate engine (they have several diff. power levels)
Good luck with your project.
Cruisin in my '73.