The "upper" is the barrel and receiver, that's why the serial number is there. The two can be separate since the barrel is threaded(?) onto the receiver. IMO, that's something that should be done by someone skilled in the art. The rest of the Ruger erector set can be modified by the user.
Looks like you got a good price on a MKII in good shape. And, yes, those rimfires are loads of fun to shoot, inexpensively and a lot less recoil. The aftermarket barrels get good reviews, but if you are looking for a stainless you might get satisfactory results at not much more by buying another Ruger in stainless. They are like peanuts, one is not enough. Check out the steel models of both the MKII and MKIII's. Different grip angle than the 22/45. The steel Rugers allow you to change grips, something that can be done only with a lot of work and modification on the 22/45. I have a MKIII Hunter, MKIII 22/45 and a MKII GC. All are very enjoyable pistols.