if you want a nice easy fuel efficient engine with decent power amount then i guess build a nissan KA24DE, very good 4cyl gets about 25mpg and can be built to put out about 250hp NA or around 400 turbo'd

if you plan on making your own block and heads.... well you're up the creek without a paddle my friend, and should be working on an F-1 team if have the no how to engineer from the block up