Maintenance is more important than anything else.

If you find a nice, clean, well maintained S4 for a decent price, buy it.

Small stuff will add up real quick, so saving a little now can end up costing more later.

I wouldn't pay a premium for low mileage with these cars. It seems that alot of small things start going around 80-100k, so getting one at ~110k miles that has already replaced them is best IMO.

Model year doesn't matter really, but all things equal, I would pick an 01.5-02 over 00-01.

If your on a budget, buying an already built car can save you a ton of time, money, and heartache, assuming your not buying someone else's nightmare.