Well, BMWs have a reputation for being great high milage bikes. Especially boxer engine models. You'll find many a diehard BMW owner out there who will tell you the boxer engines just start to hit there sweet spot at 100K miles. Not sure this is the case for the K12 inline four engine however. I'd search this site a bit for older posts where K12 owners have talked about some of their engine issues. I'm sure many will jump on this thread.
I guess in addition to knowing the typical engine issues people have had with the bike, the decision needs to also be driven by price, service history, how well it appears to have been taken care of (look for signs of rust, corrosion, etc.), how it rides (wouldn't buy without riding), and how much work needs to be done to get it in tip-top shape if things are looking or feeling worn (like springs, cables, grips, tires, etc.).
Personally, I gravitate to low milage bikes and am happy to pay the high price. That's just me. Lots of people high milage gems out there. You just need to know the typical issues people have had and really scrutinize the history and condition of the bike.
If you can, might be a good idea to speak to the dealers and service techs who has been servicing the bike over the years. They might be able to give you some insights on the bike.
"Behind every great man is a woman rolling her eyes" Jim Carrey