Can I replace carrier and motherboard without replacing memory?

Hi guys, I'm in this fix an iphone situation and can't find a good answer for my question. So I have this iphone (white 16gb) for 2,5 years and it's a bit messed up. My friend has an iphone 5 (black 32gb)which motherboard is not working. We made a deal that she sells me her iphone for parts and now I wonder if I (not me personally but the professional mobile service) just swap their motherboards and continue to use the black one will it:

1. have his original 32gb memory or the 16gb-basically is the memory part of a mother board? If it is, can I swap the memory as well?

2. If she uses a different phone carrier will putting in my motherboard just unlock it for my carrier or will I need to unlock it additionaly?

4. Will the itunes still recognize it as my phone or will it be a new device?

3. is there any problem that might happen with that that I didn't think of :D?

Sorry for sounding dumb and unprofessional but this is all rocket science to me. And thank you in advance fot answering!