What's high milage? The better question is, how has the bike been maintained and by whom? Since you know you'll have to do the cam chain tensioner regardless of the milage, I'd say an 88' is starting to get long in the tooth at 90-100 K miles. And, the cam chain tensioner has to be replaced - preferably with the H-D hydraulic unit rather than some after market gear set-up. You real should carry a spare master link on the bike, or change out the circuit breaker to a 50 amp fuse.

If you get an 88', two know issues that you'll need to address are the cam chain tensioning mechanism and the master link circuit breaker. That's my round about way of saying, the 88' five speed is the way to go, unless you're able to pop for a 103'. The 103' works great with the six speed and pulls hills with aplomb. I didn't mind it at first, but it really started to annoy me, and I couldn't keep up with my wife, who rode a 103'. So, I spent a lot of time - at altitude, on hilly roads, and against the wind - in fifth and even fourth gear with the 96'/six speed bike. You had to make RPMs to keep from bogging down and the higher gear just made the bike bog down soonere at any reasonable speeds. When the 96' was mated to the six speed, the combination was less than satisfactory. Thus, the engine is more powerful only at higher RPMs. Harley achieved the displacement of the 96' by boring out the 88'. I've owned an 88' (2003 Road King), a 96' (2008 Ultra), and now a 103' (Road Glide Ultra). IMHO, the 96' and 103' engines are really good, you can't go wrong with either, and I strongly suggest the 6-speed. Very nice bike, I just don't need another bike, or I'd buy it from her. If you weren't so far from me, I'm selling a bike for the widow of my long time friend/co-worker. I don't know how their bikes held up during the Evo motors, but since they've been buying Twin Cams (which came out in '99) they haven't had any issues, nor have I. They ride them for 5 yrs, put 100k on them, sell them to a co-worker as a starter bike (knowing they won't put as many miles on them), and go buy another new one. I have two co-workers who regularly put 20k per year on theirs. Between the two bikes, I rack up 10-12K miles per year. I also have an '11 Electra Glide with the 110' Screamin Eagle w/6 speed. It has 47K and still going strong with no issues. I've got two Twin Cams a '05 WideGlide that started as a 88', now a 95' with lots of mods.

Twin cam 96 reliability