whatever discount you want off the car, offer a sum that eqautes to double that discount off. You then stand a better chance of meeting your original discount, since more often than not a sale is done by both parties meeting in the middle.
Doesn't always work, but worth a try
Nothing wrong with non MB dealer as long as fsh etc. Take someone along with you that knows about the car, he can point out faults, which you then can use as levering points to neogtiate a bigger discount, or walk away from them if they are bad
Rule of thumb, I would expect to get at a very min 5% off but would think I could maybe end up with nearer 10% off. Therefore make your first offer about 20% off. Tell him that this is all the money you have, and be prepared to walk away if he does not entertain you. You can always go back later, he would only be too pleased to see you again.
However :
Golde rule no1. Do your homework first. Use Autotrader etc to find very similar cars, find their prices from the dealers advertsing in there. Remember these are advertised prices, the cars always go for lower (as above). Get an average price from these. This will then give you a feel of what the car is worth
Golden rule no2, never ever be embarrassed about offering a what you might think as a 'silly opening price'. At the end of the day it is only a dealer, someone you probably dont even know. It is his job to talk to you, however always be polite, worse case scernerio then is you are just wasting your own time if you go in far too low. I bet he did not feel any embarrassment when he bought the car off the old owner for 20%-25% less than the current advertised price !!
