I imagine you cannot take the AGA out of a A1200 and put it in the 1500 - that would be far too simple.
Yep that would be far to simple but would be nice if it could be done
Well if you are after a AGA Amiga 1200 you could always purchase a refurbished one from amigakit.com for 117.43 GBP, with the options to upgrade the Rom etc. Also check Amibay.com you will find prices alot more resonable

avoid ebay if you can, and I wish you luck.
Or you can even run an Amiga emulated on Windows (WINUAE)
http://www.winuae.net/