Would the owner allow you to try it at a local range (and he could try the .22 rifles) before the trade?
While I also like the 99 series pistols I didn't care for the SW99 45 model. I say that after having spent some range sessions shooting one during some T&E a few years ago.
It was acceptably accurate. It functioned normally with the JHP loads used.
It's just that the gun felt a bit big & unwieldy for a plastic .45 and the slide's cycling was less energetic (and the ammunition being used didn't cause this same effect in the other .45's being used). While I'm not particularly interested in the G21/21SF or the USP 45, I actually preferred shooting those models to the SW99 45.
I had been a bit eager to see and try this model SW99, being a .45 owner/user as well as already being a SW99 40 & SW99 9c owner. I was a little disappointed this gun didn't seem to handle and shoot as well in .45 as it did in .40 S&W and 9mm.
That's just a subjective opinion, though. You ought to try it for yourself.
I think it was in my third SW99 armorer class that we were given some background info about the pending release of the SW99 45 and how S&W had really had to work with Walther to get them to design the larger frame for the project. We were told that Walther apparently had no interest in producing their 99 series chambered in the American .45 ACP caliber and had resisted developing the larger frame for a while. The SW99 45 would be a model for which there would be no equivalent Walther model.
I suspect that S&W didn't put as much effort into the SW99 45 because of the development of the M&P pistol project. The M&P project gave them the opportunity to make significant changes when it came to a plastic framed pistol without having to work with another company's ideas of what a particular model design should have in it.
Now, as I mentioned earlier, I happen to like the 99 series models. I carried an issued SW99 40 for several years, and I still own a similar SW99 40 and a SW99 9c (both in the traditional double action, or DA/SA, version).
As with the Glock, I have a personal preference for the 99 series when it's chambered in 9mm, myself. My SW99 9c is a fine little plastic 9mm pistol, and I like it well enough that I have no plans to add a M&P 9c to my collection of 'working' compact 9mm's.
Now, as far as plastic .45's go, I've owned a M&P 45 for about the last couple of years and it's demonstrated itself to be a rather surprisingly good pistol. I like it quite a bit. Enough so that I wouldn't mind having a M&P 45c sometime, just for the half inch shorter slide/barrel and the almost inch less height in the grip frame (.97", I think?).
I own 9 pistols chambered in .45, and the M&P 45 is the only plastic .45 which really interested me enough to buy one, and now I'm actually thinking about picking up another one. Go figure.
Sorry if none of this helps you with your choice, as I tend to ramble a bit in some thread postings.
Try it if you can before making the trade. See how you like it.