$300 is a great deal, they were both $1000+ new, particularly the 4100dn.
The maintenance kit includes a fuser, but there is much more to it. It also includes feed/separation rollers for each tray, a feed roller + separation pad for the manual feed tray, and a transfer roller. And no, they aren't terribly difficult to install.
Again though it's 110K pages away so don't worry about until it asks for it.
One minor thing to look for on the 4000, if you pull the toner cartridge out you'll see a line where metal meets plastic. Is that line straight or does it have serrations (think of a ruffles chip)? If it's straight, it will work fine just make sure you don't tear anything off inside there. There was a newer style on later 4000's (and all 4050's) that provided better access to the paper path, with the old style it's possible to jam it bad enough to require service.
If I ran across the same deal I'd have jumped on it. They aren't that fast by today's standards but they are real workhorse printers.
Viper GTS