Sounds like a good buy. I assume it comes with all records? If so, I'd go for it. Offer him $10k and settle half-way (?)
At around 100-150k miles, the timing chain/rails should be replaced. Since you're in there, it'll probably need valve stem seals, maybe rocker arms and possibly and a cam or two. Top ends on these cars need careful attention!
A/C repairs can be pricey, $800+.
In regards to winter, you could always buy a set of winter skins. Never done that myself, since winters are relatively mild in southern WI.
When you drive it, pay attention to trans shifting, smooth? Any smoke at start, vibrations through the steering-wheel or brake pedal?
Oil leaks? Rear mains tend to weep -- maybe not at 69k miles, but it will.
I don't think these cars are shop magnets. They have relatively old technology, not all the gizmos like the W140s. Relatively easy to work on, parts prices are reasonable, what else can I say. I love mine and cost of ownership hasn't been much more than other cars I've owned.
*Edit: MikeTangas and Thomaspin are the resident 560SEL gurus on this forum. Over the years they have provided a WEALTH of information on their cars -- and be sure to check out Thomaspin's web site HERE
and perform a search on their posts.
Best of luck!