I just bought a 178k Bitdi for £6k. It's got a full history with oil changes every 10k miles but gearbox oil not changed (or at least can't find any record). Drives like a dream and in great condition. Spoke to owner who had it from registration until 2021 and also his mechanic who looked after it once out of warranty. Been through 2 others in quick succession with less miles but again service schedule maintained. So it's a good car with a good provenance.

Last guy did rear air suspension, serviced and gave it new tyres. This car has been loved!

But I am terrified the thing is going to crap out - I have read too many horror stories about turbos, compressors, gearboxes, DPFs and EGRs and that it's just a matter of time.

Am I mad? would you guys have done it?

All those components you've mentioned could fail but if it's been well maintained then that reduces the risk. What major parts has it had changed, do you know? Might be worth asking the original owner or the mechanic if any of those parts have been changed except for the air suspension (I'm assuming hu changed the rear suspension air bags, and not the compressor?). 

Lot of car for the money. Personally, I’d opt to change the oil every 6-8k miles. This will be a Tiptronic transmission 8-speed and generally bullet-proof, but again, if it were me, get the fluid changed. Yes, Audi will tell you it’s sealed for life, but ZF who make them have a service kit. Turbos, your guess is as good as mine. Don’t hammer it, and if you do, make sure it’s warmed up.

Great cars, the key is servicing with the recommended oil. If that’s done you shouldn’t have any major issues like timing chain. My A6 C7 Allroad 245hp is on 185,000 miles with no major issues. I bought it at 165k with full service history including gearbox service. I change the oil at +/- 5000 miles. If it doesn’t have gearbox service record then get it done immediately.