The problem with saying 'well maintained' is that it doesn't specify what type of maintenance. Some guys wash their car every weekend (which is great) but leave the brake fluid in for 10 years (which is not great).

On that car, what you want to worry about is trans fluid changes, bevel gear changes, engine oil service (changes) to take care of the turbo, and engine oil type and/or PCV system maintenance. If THOSE items are included when you say 'well maintained' there should be nothing wrong with this car that you can't see with your eyes.
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1998 V70 AWD Turbo 195k+

And my 1998 V70R with AWD was great. Of course, the bevel gear fluid needed changing and the vicous coupling cmplained at low speeds. Other than that, just normal Volvo FWD proglems.
Leaking oil??? From where? If the rear main was replaced and the oil separator was not, then probably an oil leak at the rear cam seal. And probably a leak in the turbo return seal. I solved a leak from the weep hole in the intercooler by drilling a 1 inch hole in the air guide under the car.
The bevel gear falling apart is usually due to neglect. The fluid does need to be changed every 50K miles, as does the tranny fluid in your S70.
If the car is maintained, the 1998 is a good car. The only deal is the AC evaporator, which is very expensive to replace.
If the price is right and you feel up to replacing the oil separator and the turbo drain seal...

Klaus
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The more I learn, the more I forget. So why learn?

I am a Volvo tech at an independent specialist. The 1998 V70XC pays my mortgage. Known lemon. Stay away. Volvo's first attempt at AWD and it wasn't a good one. Eats front end parts, leaks oil if not maintained, electrical problems, catastrophic failure of AWD components regardless of tire tread depth. Nothing like your naturally aspirated two wheel drive 98 V70 which is one of my favorites. Model year 2002 and beyond for V70XC.