I would have offered $350 - which is a great deal. If you had to pay close to the asking you still did OK, but I would think you can get it for $375 - $400. To me and from what I can see in your picture, the gun was shot, but carried more. It does not look abused and M15's are work horses, reliable and accurate. With a little care and common sense they will last lifetimes.
As excited as you are, check the B/C Gap, lockup, timing, end shake, push off, tightness and alignment of the Yoke and make sure the ejector rod is not bent. Also make sure the sights are not wobbling and that they are not chipped. Look down the barrel and in each cylinder hole for rust and defects. If you do not know how to do these simple and easy checks, bring someone with you who does. They should not take more than 5 minutes. Do not rely on the seller to tell you how it's in perfect shape how great a deal it is.
Buying a used vintage revolver can be very rewarding, satisfying, and getting a quality made piece of history no longer produced these days, but you must do your diligence so you do not get a case of buyer's remorse. Good luck on Monday! Let us know how you made out.
NOTE: IMHO new production S&W revolvers are not comparable to vintage models. The only disadvantage to vintage is that parts are getting harder to find in good shape and there is no warranty. Still, I only buy vintage S&W revolvers prior to the mid 1990's, mostly earlier. Thankfully I have a plethora of parts and know how to repair & work on them. The good news is that a good vintage gun should last a long long time with no issues (providing it was good at purchase)!