Congratulations on purchasing your 01 M5. Good idea to get the best R.D. on the market today.
The $2,700.00 someone wants you to spend is unrealistic because a Valentine One, Stealth One module and the Lidatek LE30 only cost $1,000.00 plus shipping and installation.
Go to the Valentine One website:
http://www.valentine1.com/
The unit costs: $399.00 plus shipping.
Secondly go to the Stealth One website:
http://www.jaricdesign.com/products.htm
Buy the remote module for $199.00 plus shipping.
Lastly, the Lidatek LE-30 site is:
http://www.lidatek.com/
The LE-30 unit is $399.00 plus shipping.
Plan to spend 2 hours wiring the Stealth One unit into your Beast and maybe another 3 hours installing the LE-30. I'm not an electrical engineer or tech, but the instructions are very clear.
The worst case is you find a local reputable audio shop that can easily install the Stealth One and LE-30.
I hard wired V-1's in my three current vehicles and have not regretted shelling out for these insurance policies for one moment.
The Stealth One display in the M5 is so well thought out that it is a bargain for $200.00.
I hope this helps.