I'm gonna highly suggest you pony up the cash and have a powow with a machinist. He'll set you on the right track.
I dont mean to be a dick I'm just trying to stop something bad from happening but assembling a bottom end seems like it might be, how do I say this politely, out of your mechanical range.
Find a good machinist who will tank, bore, and hone the block. That shouldnt cost more than $300. Tell him your interested in learning and you'd really like to assist in the assembly or at least watch and learn. If you find a good guy it shouldnt be a problem. Theres no reason he cant put you on the right track as far as what pistons you need, whether it needs to be bored over, etc.
As far as your original question. Piston selection completely depends on desired compression ratio, head volume, price range, etc.