For Suspension, The front upper and lower control arms will have play in them after 75000 miles. They cost about $250 per side and are easy to replace yourself. I would check the CV boots and bushings. I would imagine that the bushings would be squishy after so many miles.
The tranny is an odd thing for these cars to have a problem with. Plus you can't rebuild the tranny because the parts are not available. I think that I had some rotten luck and the previous 2 owners beat the shit out of the car. Currently, my tranny is working okay except that it grinds when I shift into 4th gear. Eventually, I won't be able to shift into that gear. When that occurs, I will replace the tranny. The cost is hefty for the manual, new from BMW, it will run $5000 plus labor. Used, maybe $2400 plus labor. When I have to replace my tranny, I will buy an M5 tranny.
Overall, my yearly cost for maintenance is oil changes, brake pads and brake flush, $600. It is hard to predict what might catch up to you in any given year.
Does the AC work in this car? Does it have heated seats? Sometimes the heated seats on the 540 don't work.