Looking at buying a used a4 early 2020 model (that’s all my budget can stretch to) is the recommended time for a timing belt change 35k miles or 5 years whichever comes first? 
I assume a gearbox oil service is the same as well. 

Google isn’t really helping. 

Yep you're right 5 years. Audi did change it to unlimited age and 120k but this would be unwise to follow. As it used to be 5 years. Any car you're looking at , it's worth asking if the supplying dealer will fit a cambelt and water pump. 

Definitely worth looking at gearbox oil services. I think the mileage is 50-60k I think. But definitely worth getting it done for peace of mind. 

Don't forget to check out buyers guides on YouTube. 

Thank you

Apologies for jumping in Steve.

I very much follow Steve’s views on this - unlimited time interval is asking for trouble. However, I would guess ‘the trade’ would (understandably?) following Audi’s current recommendation, and it’s unlikely that they would fit a new belt (this engine is belt driven at that time?) ‘within the deal’.

Taking this as a strong possibility, and appreciating the car is not yet 5 year old, I would suggest you wait until it is, then decide who you going to use to fit it, and most importantly, what make of parts would ensure a good job is done. 
We can give you additional advice if you need it.

Regards,

Gareth. 

Thank you