I've owned 5cyl Audis before; but for the most part, I've been around B5s since 2OO5. Any particular prepurchase suggestions, areas of concern, outside of the typical used-car stuff specific to a CGT?

If you're going to get one, get one in as good condition as you can afford, because you'll likely want to keep it a lot longer than you'd imagine. Low mileage, minimal rust. At 160k mine's starting to rust in some of the usual areas: top of the rear wheel arches, door sills, and my front right jack point is getting soft - so look at those areas. Mechanically, the engine I think just goes forever. I'm still on the original clutch. Other mechanical stuff I've done to improve performance rather than because anything broke (although I did just need to replace my alternator - the most expensive non-performance thing I've done). Other items would be the side window regulators need replacing periodically and a big issue is the digital dash on the 86.5 and later cars, for which no-one in the country wants to touch. If it has one, make sure it works.
I'm sure others here will have a lot of more specific advice.

Weatherstrips are getting hard to find. Window regulators are known to fail and are expensive. I've got a how-to on my site to replace and maybe fix the problem. Post '84 has better wiring and a nicer dash, but the stock aero headlamps suck. '87 with the 2.3 and rear disc brakes is the best of them all. Avoid the automatics; they are dogs. Watch for rust around the rear wheels, in the trunk wells and the door bottoms. Drivetrains are bulletproof.