Reliability is good, mines done 70K in all weathers.. OK that's in the south but there's still snow down here and we have had some deep floods (wettest I've ever got not swimming!) personal experience says with high level pipes the motor runs happily underwater
Servicing on the 2 valves is mostly regular oil changes at 3-4K and belts every year. The belts are peasy, once you've got the hang of it it's wrapped up in under 30 minutes.
Valve clearances are a PIA but you don't need to do that often. Usually once set they stay set for 20+K.
The water cooled S4's are a real hoot, well they would be with that motor!!
Go for it, just remember to get a good service history or one that has been looked after (oil and belts especially). If it's a newer injection model make sure you get all the keys!!!