I have a '98 the headlight problem is fixed by a re-call so I would not worry about that. Spark plugs I change myself so I don't see that as a big cost(note it easy to change the plugs as the engine is in line, the non turbo is a little easier then the turbo, also I think '99+ have a distributer-less ignition). The expensive thing on the s70 is the ABS module @ $500 which seems to fail for everyone (I think this is an issue of the 850's also, search the board for more info). The other common failure is the upper engine stablizer mount, it is easy to replace and I used the poly-u one that IPD sells (so now it should outlast the rest of the car).
My fealing is that brake rotors are usually a comfort thing, on every car I have owned (Chrysler, GM and Volvo) I have had to change rotors at 45-60k because of vibration/warping not because of wear to min thickness.
The other cost big cost is tires, they are expensive for the t5.
Overall I am very happy with my '98 s70 T5m, it has a really sweet engine.

Good luck
'98T5M