I rarely use flash with analog gear. That said, my F3 does take non-TTL ISO flashes without adapters. Granted, it is the Press version. Paid the same for it that I would've paid for a regular F3HP in similar condition. $165 shipped. If you're looking for flash capability, look into an FE2 or an F4... Both are similarly cheap and are faster than an FE, flash synch-wise (note the 1/125 on the FM/FE, and the Nikkormats is actually 1/100 on a shutter speed tester, so you really aren't getting a faster synch speed over an F2 or an F3 - at least, not enough to make a real difference), being at 1/250.
-J