Great score! Mousin' Around is one of the neatest System 11s and this one looks beautiful! I'm sure it'll clean up nicely.
I've had a few freebies years ago, when pins weren't as expensive as they are now. A basket case Stern Big Game and a very nice Bally Embryon are the ones I can think of.
Apart from freebies, I've had quite a few that were what I consider almost free: a 50€ Fireball II, 350€ Banzai Run and a 1250€ The Addams Family come to mind.