BORROW a Mac!! And, buy an (inexpensive) PocketDrive to backup its working system and contents onto - using "SuperDuper." When you've bought the latest iMac, and need to to return the borrowed Mac. Simply clone the PocketDrive clone to the iMac. This assumes Intel to Intel hardware. If Power PC to Intel scenario just clone the data to the iMac Intel.
My apologies, I should have thought before I posted...
You ARE up against it.
Go with the iMac you can obtain.
You can always sell on rapidly the "fossil iMac (2006)" to some "sucker" at $100 less than list and then buy the "post-fossil" iMac... at the new reduced price!