Have owned an RS5 and driven the RS3.
Generally speaking they are pretty reliable but don’t modify them unless you want headaches. There is no unicorn dust in them so servicing at a cheap place is fine. RS3 is young enough to not give you headaches buying used. If buying new I would suggest a used C63 all day long over an RS3. I have a BMW M5 and my wife drives a C63. No comparison to the little cars.