I'm looking at a 1996 Ultra SKS 700cc

Any problems associated with this one?

As with any Ultra, get a set of triple pipes for it. That alone will net you anywhere from 34 to 44hp depending on whose pipe set you get. Most remain pump gas reliable, and SLPs are actually pretty quiet.

You'll want reverse.

Everyone else will complain about the heavy chassis, but with 150hp on tap, everything's light.

I had a 96 sp and I loved it, almost regret parting it. Almost. Direct to your question. High milers can have issues with the PTO bearing freezing and spinning on the crank (you can hear it from time to time on shut down). Also, 96 model year used an oil bath water pump independant from the crankcase. 97 and later lubed the water pump gears with injection oil. Not that big a deal. Some complained of the internal seals going and dumping the oil. Just fouls the center plug. Mine had over 6000 miles on it and was fine. Be sure to check that the sight glass is still in the oil pump. Again, I have read that guys had them drop out, they can be bought as separate items. I never had an issue.

When it's all said and done that 680 was the basis for the XCR series. It can be big bored to 900 and be very reliable. If it's an excellent deal (sled in good to great shape for under 2g's with mid mileage) don't let anyone talk you out of it.

Also, all Ultras and most XCRs will break the PTO side motor mount. Crack it right in half. Lift the clutch cover and push the motor to the rear. Look at the motor strap right at the front side base under the clutch for a crack. It's a $40 dollar part, but a WMS torque arm mostly elimnates the breakage.

Check all skid bolts to be sure they didn't loosen up and open up the holes. Standard xtra 10 stuff.

If anything it'll make a reliable cheapo backup sled, that with a little money can be a mean runner.