HFM Control Unit or OVP???

My wife's 96 c220 won't start. It cranks but doesn't fire; it just turns and turns and turns but doesn't ever spark. I tried everything in the manual about start lock and locking/unlocking with/without the remote/regular key in both the door and the trunk...nothing helps. So I had it towed to the dealership and after checking EVERY wiring point in the car (so they say) they want to replace the HFM Control Unit for $1400 (just for the part). Everything I've read on the site so far indicates it would be the OVP. Is there a way to diagnose one vs. the other? I've read the posts about the different OVP part #'s...when I asked the dealership to check that out they didn't have a clue what I was talking about. Thanks