Well the 3S-GTE engines should eb easy to find on ebay but they are getting old. They haven't made them in a good number of years so parts might start becoming scarse. I think you'd have much better luck going with the Mitsu's EVO engine, it's newer and more efficient as well as more available.
My guess is 10,000-15,000 on the 3S-GTE, the EVO engine will cost more off the back but excluding the cost of the enige you could reach 350-450 for under 10k for sure. It all depends on the parts you go with. Also it would depend on how much custom fabrication may or may not be needed to get the engine to fit in the chasis you are using.