Jack,

 The 1990 Olds 98 is the last year of the first-generation FWD GM "C"
bodies. By then they got almost all the bugs out of the design. If, by
chance, that car is a Touring Sedan (VIN would start 1G3CV . . .) then
grab that car - they are great vehicles. Even the regular 98 of that
vintage is a good car.

 Your gas mileage won't be quite as good as the Century - heck,
you're pushing around nearly 500 pounds more steel with that 98. On my
'87 98 I got around 18 city and 25 highway..

 Biggest potential problem: the automatic transaxle. If not properly
taken care of, those 440T4 (4T60) transaxles last around 120K before
needing a rebuild. If you can, drive the car and see how the transaxle
responds. Telltale signs are delayed engagement when shifted to R or D
when cold, delayed upshifts from 1st gear when cold, a shutter when
the torque converter clutch engages.

 The only engine issue worth noting is common to all early 3.8L V-6s:
timing gears wear out, requiring replacement of gears and chain. If
done in a shop look at around $600. Once replaced they are good
forever (the replacement ones are all metal - makes the engine a bit
noiser but more reliable).

 Most of the other troublesome items with those cars are issues like
the power antennas (GM doesn't stock replacement parts anymore), most
trim items are NLA, rocker panel rustout in rust prone areas of the
country. I can't really think of any other "show stopper" issues that
are common. If the car has ABS the system they used then (the Teves
Mark III) works well but costs an arm and a leg to repair if it fails
(example: ABS pump assembly is $800+, and master cylinder assembly is
$1500+).

 Other than the items above I'd look the car over real carefully and
have it inspected by a good mechanic that knows GM cars if possible.
But if the car is in good shape you'll have yourself a nice ride.

Regards,
 Bill Bowen
 Sacramento, CA