2006 Suspension
 

I'm considering buying a used M5 that has about 100k miles on it. It's in OK condition and I think I can get a good deal. When I drove it the suspension was terrible! The whole car seems to float and everything feels really loose. I do a lot of my own car work, so I was thinking that replacing the front struts and rear shocks should clear most of the issue up. Are there any major issues I'm not considering? I assumed I would get the aftermarket KYB hardware.

The OE suspensions on these (especially the rear) are not very good. Aftermarket is the way to go.
With something with that many miles, if you can't get service history, i would change ALL of the fluids. Change the fan belts, clean the throttle body, MAF, change the PCV valve, clean the EGR valve.
You may have to change plugs, coils as well soon.

Thanks for the quick response. I've read on these cars a bit, so I'm not walking in totally blind, but it's easy to assume the work to be done is easier than reality.

I have a 2007 and replaced the struts and shocks myself. Wasn't to hard to do. I went with top line KYBs all around. I would also check the front and rear sway bar bushings. They will creak loudly when it is time for them to be replaced. Rears are very easy, but the front bushings were a pain.