I've spent the afternoon on ebay looking for some parts - came across a couple of auctions listing a "7/8 speed Dura Ace rear derailleur."

Are 8-speed derailleurs safe to use on my 7-speed Trek? Or do I need to stick to specifically 7-speed replacements?

Yes absolutely safe...No problems. Old derailleurs can even shift 9/10speed gear.

For a pre-97 DA derailleur (RD-740x), only safe if you're running DA shifters. 105/600 (Ultegra) are a different geometry.

Thanks, avenan. I'll be keeping the 105 shifters.

So basically I'm not limited to the 7-speed stuff, as long as I stick to 105 or 600?

That's correct. There were style changes, but the derailleur geometry remained (remains) the same. DA changed with the switch to 9 speeds, so a 9/10 speed DA derailleur *will* work.