1x dead iPhone 4, 1x locking up iPhone 4 help

Morning, Just recently i thought id got a bargain on ebay regarding a pair of iphone 4.

The first was described as working correctly, can make / receive calls and send texts etc but randomly locks up.

Ive got the phone today, unfortunately its locked to the 3 network so ive got a micro sim on the way so I cant actually proceed further at this point. Although it did crash once on me when i tried to select language's etc - had to hold the home and power button down until it turned off.

Ive read on the forums that this can be a common fault, and can occur to any iphone - faulty or otherwise. And the main concensus seems to be to take it back to the store for a swap if under warranty ( which theyre not ).

My main question is - seeing how im likely to come across this fault again - is the fault software or hardware related?

If its hardware related what circuit board is likely to be the culprit? If we could identify the problem then im sure I could just order the required part on ebay.

The second iphone 4 white - bought cheap as its totally dead. Tried a new battery and still nothing. Got a new dock / charging connector on the way - is it likely to be this at fault? Doesnt seem to be any sign of water damage, and it wasnt described as this on the ebay page.

When id swapped the new battery over i tried to charge the phone ( to see if id fixed it ) and it got extremely hot - so i disconnected right away and left it to cool off.

Whats likely to be the issue with that one?

Thanks