yeah older german cars are tanks, and usually very reliable. that era of audi's are in most cases, are very reliable cars.
honestly any car you buy for 800$ is not going to be reliable. but for an awd car, id trust an old audi over an old subaru easy. the cars are just way more overbuilt and robust. but before you buy it, have a mechanic check it out, have him tell you whats wrong with it, and what it needs to run reliably. buying a car with known problems is never a good idea. but if you buy it planning on fixing any issues right away then id say go for it.
just really have a mechanic check it out first to make sure its not gonna cost you twice your budget.