The E34 is better than 75% of the cars on the road today. You'll be happy with any one of the models you listed.
I own an M50 and an M60. They are both excellent cars. The transmissions are workhorses. The M50 has very little "get up and go" -- low on HP and torque. The M60 is a beast... I don't mind spending $50 a week on gas.
Try to find a car with less than 125K on it and with all maintenance records.
Do a forum search and a google web search for things to check for... you'll find a lot of info.
The trick is to find one that's been maintained properly and looks good cosmetically. Ideally, you'll find a 94 or 95 540 in excellent condition. Check the classifieds... there are two 540 for sale. One is an auto and the other is a 6-speed manual.
1994 540ia|Oxford Green Metallic|Lemforder|KYB in H&R|18" OE Staggered M-Parallel with Pirelli P-Zero