I am helping an acquaintance get his M5 ready for sale, and one of the things the camera needed was its battery contacts replaced, as the contacts were broken due to corrosion. I have sent the camera to DAG and the battery contact replacement is costing $50, plus $20 to calibrate the meter for modern batteries.
A full CLA from DAG or Sherry will cost you about $300, which is about what they charge for other M-series cameras. The camera you purchase on e-bay may not need a full CLA, but it is a risk that you are taking. If I were looking for an M5 at a bargain price, I might be tempted to do exactly what you are doing.
If you are interested in buying a v. nice chrome M5 (3-lug, everything working to spec), PM me. I'm going to put it up for sale as soon as it returns from DAG.