i'm actually going to play devils advocate and say an 100K mile N54 is a great bargain for the money, even considering maintenance cost. BUT, I personally wouldn't consider buying one that hasn't had the turbos replaced under warranty fairly recently. Look hard and you can find a 100K N54 with recently replaced turbos for essentially the same, cheap price as most 100K miles N54.
Honestly, I would not buy any N54 without recently replaced turbos, regardless of miles, 40K, 50K, 60K, if it hasn't recently had replaced tubos as the 8 years of warranty are running out on most N54's turbos now. A 60K N54 still has 60K on the turbos and they will go out, and there is a strong chance it will be out of warranty by then (if its not already) cause its 2017 now. and a 2010 N54 car with 60K miles or less will go for 5-7K more than an older one with 100K miles. Turbos are a lot less than 5-7K, even upgraded stage 2 ones
I got mine at 96K miles with dealership receipts of new turbos at 81K miles, I paid pretty much what any other N54 135i would cost with 96K miles, it just took me awhile to find my car. Good luck OP