The 2.4L is a great engine. The problem you may find is that the wideblock version is pretty rare. If your not in a rush and don't mind traveling to a bunch of junkyards to find one it's not a bad choice. I do doubt that building up a 2.4L costs any less than building a solid 2.6L or doing a V6/V8 swap.

I just bought a rebuilt turbo 2.6L for $400, put another few hundred in it making it "right" and now I know the engine in my truck is good for at least another 100k. I did cheat a little by using a lot of the old stock parts off my Conquest that is now converted to MPI. That got me a bunch of stuff for "free". Imho, buying & building even a mild turbo 2.4L for less than $1000 would be hard to do.

The domestic V6/V8's are a great choice too, but be prepared for a lot of work, fabrication, welding, etc.