You can inspect the subframe pretty well, just by getting under the car and looking. Technically, the formal inspection involves removing the two rear bolts in the rear axle carrier enough so you can see above the bushing...but you need to support the rear end to do that before lowering it.
Belts you should put on anyway. But, if you have a running M5, do you really want to throw the dice on a car you've had for 2 weeks and drive it to St. Louis? Actually, the odds are, it would make it...but you can increase the odds substantially with good PPI.
Let me know if you want me to find things wrong with the car you want to buy!