$.02 here. If you are on a tight budget, defer the mirror. Get a set of LED turn signals and bar mounted mirrors. The bike will look better(in some eyes but not all), and you can get mirrors and turn signals on eBay for under $30 total. Then you can ride while you look for another mirror to show up for a reasonable price if you still want one.
Before you go full replace on the rubber bits, disassemble the bike and do a careful inspection of them. I bought a '91 RS a couple years ago that had sat for several years, and found that the only rubber bits that had perished were the fuel lines.
Check with the local auto parts store for a fuel pump. The Ford Mustang is reported to use a similar pump and they can be had for about $60. Chinese parts are getting better now. I have an $11 rear master cylinder on my touring bike that has 8,000 miles on it and works better than the OEM unit ever did. I think Martin has a Chinese pump on his bike and has had good service from it. The prices are low enough that you can buy two and carry one as a spare(I do that when I'm out West in Montana and Wyoming).
eBay is a good source of used parts as there are a number of bike breakers selling there. These are large outfits who do a lot of business and sell good quality parts. You are especially fortunate in that there are at least two that I know of in Colorado that I have bought parts from. They are close enough that you can save on shipping costs.
Good luck and enjoy the project!
I love the integrated approach designers of '80's & '90's faired bikes took (hence the R100RS & 900SS in my sig line), and one aspect of this integrated design that I see as crucial to the whole is the design of the turn signals. I love those of the two above-mentioned bikes, and how they're bespoke, conforming to the lines of the surface in which they're mounted-- much like those of the K100RS. I hate the way modern designers design a fairing and then just tack on parts-bin turn signals, so I'll be patient. Good things come to those who don't get in too much of a hurry.
After reading rehab info on at least three forums (fora?), I've come to the conclusion that one should at the very least replace all fuel & vacuum hoses. There's a good chance the coolant hoses are ok, but my need for peace-of-mind will prevail over thrift when dealing with such critical components (or mirrors...): any doubt, change it out. Same goes for fork seals: I remember the big oil stain on the driveway after my dad's Rickman blew its fork seals, and that's not something I want to see in a driveway in Wheatland, WY on a Sunday afternoon.
I've read multiple fuel pump threads here and on the K100 forum, and have become aware of several alternative fuel pumps (as I mentioned above), but changing a fuel pump 20 miles south of Chugwater holds no appeal to me, no matter how cheap I bought both pumps. When my old Chevy S10 was rear-ended, I took the opportunity to change out the in-tank fuel pump before we mounted the replacement bed. I bought the factory pump at triple the price of the cheapest alternative, and never regretted the money spent. It seems the older I get, the more valuable peace-of-mind becomes.
As for eBay, I scrolled through RS parts the other night until I got cross-eyed. Found some good stuff (a few final drives, an engine spoiler, and some side panels to replace the sloppily repaired left one I have), but no mirrors that weren't beat up from the feet up. I'll keep looking.
We have a motorcycle salvage yard here in town, but his hours are like caddy day at Bushwood: open for like 15 minutes on Sunday afternoon. I'm not even sure he opens at all these days. Consequently, I haven't stopped in, even though my Benelli needs several pieces. Could you pass along the names of those salvage outfits you mention? It might be worth stopping in.
Thank you very much for your input, and please don't misinterpret my responses as out-of-hand dismissals: I've rehabbed everything from Fiat 500C's to Jeep M715's to Cub Cadets, and maybe have become set in my ways. I do appreciate the input, though: I'm just getting my K100 knowledge base started.