Not aware of any issues with C8 platform. Biggest thing is budget.
50TDI (3 litre) Quattro models are paired with 8 speed Tiptronic unit. These are regarded as bomb-proof. Gearbox is sealed for life, but in reality a gear oil change at 80k miles or 8 years is recommended.
40TDI (2 litre) comes with 7 speed S Tronic unit and needs serviced every 38k miles.
On both cars, if I was doing high mileage, then Tiptronic box is the one to go for. That will eat up the miles. In addition, avoid long service regimes. Change oil and filter every 8-9k, which in your case is every 6 months.
Extras I’d be looking for as standard given £40k + spend, LED Matrix headlights, rear view camera, adaptive cruise.