Like what Stu said. If you are not stuck on running a class or something, there really isn't a draw back on running a on-center. The intakes are out there and if you don't have a ported stock intake, you won't be that far off $$ wise for a on-center intake. There is a lot more inexpensive parts to build a on-center engine that will offset the cost of the intake. I would look for a wet sump on-center block, you will save the $$ on plumbing up a external pump for starters. If you build a off-center engine, that will really narrow the search if you ever need another block for one reason or another. The off center stuff really demands top dollar.
Any S2 engine with forged internals can pretty much make over 1000 HP, depending on cam/heads/turbo so a mild build will not even come close to that. You can also use some cranks that have been turned down to BBC or SBC rod jounals and that will save you some big $$ if you are not looking to make big power.