Good testers aren't cheap. I'd recommend a name brand one. Harbor freight is not name brand.
Doesn't need to be snap on but at least a craftsman or one from a napa.
Any leak down will work as long as it seals. But if it runs fine like you say you may not have a leak.
I honestly think you need a ported stock set first.
Going by cc isn't really a good gauge on the setup.
A ported stock stroke can be plenty fast. I've seen a 370 motor fly, can hang with my 4 mil no problem.
I wouldn't make a purchase like that over ebay.
There's questions that need to be answered and always something else needed. A sponsor that you talk to will ask you or say what else you need. Can port it too. Get you the correct domes.
And it's always nice to have someone to call and ask questions and theyll know exactly what your talking about