If the vehicle is operated in an area with a corrosive salt treatment to melt ice and snow, you can try spraying the upper control arm pinch/through bolt with a penetrating lubricant and some moderate heat. If the bolt is seized and eventually breaks when you try to remove it, it will need to be removed by a competent machine shop. Search the forum for methods to saw through the bolt with a hacksaw or a procedure to drill out the old bolt.

Why are you replacing the hub? The hubs only need to be replaced if they are bent or damaged from a very bad wheel bearing. The bearings are separate from the hubs & not a complete assembly like most vehicles.
Sounds like you might just need a bearing.

The hub was damaged somehow by either American Tire or this mechanic I brought it to for a state certification inspection. Wheel bolt threads were cross threaded and he had to break one off to get the wheel off.

So now I'm driving around on 4 wheel bolts and nerves of steel.

Ok , you will need the hub & bearing then. The bearing gets damaged from hub removal.

I was just shown partsgeek.com and they have a oem hub for $224, second one down.
They say it's oem but how do we know.
00 2000 Audi A4 Quattro Wheel Hub - Driveshaft & Axle - Genuine, Ruville, Front, Rear - PartsGeek

and a bearing. Again, they say it's oem but ecstuning doesn't label this brand as oem.
00 2000 Audi A4 Quattro Wheel Hub - Driveshaft & Axle - Genuine, Ruville, Front, Rear - PartsGeek

Ecs tuning comes with the bolt and 4 nuts. What are those 4 nuts for?
Audi B5 A4 Quattro 1.8T Search SiteSearch Wheel Bearing ES#260993 Front Wheel Bearing Kit - Priced Each (75mm) - 4B0498625
and a bearing for

The black nuts look like the nuts which hold the two lower control arms to the spindle.
The silver-colored nuts should be used for the upper pinch bolt and the TRE pinch bolt.

Aftermarket hubs are like 1/4 of the price, with that much difference it makes no sense to go OEM.

It's tempting but I imagine there is a difference in the process that maybe leaves the cheaper one not as robust?

It's definitely not the same quality, but do you really need a hub that'll last 300k+? I'm sure one that lasts even half that time will be perfectly fine since your car won't be on the road forever. I would think that the vast majority of hubs that need replacement do so either because someone drove on a bad bearing for too long, or someone damaged it while changing a bearing. If you avoid doing those two chances are it'll last the life of the car.

You can get a MAPP system at the big box store for $40 in the plumber's propane torch section. It will burn the boots off of your upper control arms since you must heat the hell out of the pinch bolt tunnel. You can probably drive around with the burned boots for awhile but then you are looking at replacing control arms later.