I was wondering if anybody could advise me on the relative merits and disadvantages of working with older macs.

I was shopping for used powermacs on craigslist.

Let's say I was able to pick up a PowerMac G5 1.6Ghz (single) with 1Gb RAM...Am I going to be able to reasonably edit a 90-minute project in ProRes?

I understand there are different compression settings for ProRes, including an offline codec--ProRes 422 (Proxy).

How old of a mac can you get while still remaining viable?

Maybe. A dual G5 certainly can handle ProRes but a single? Not so sure it won't be an adventure in low resolution preview editing.