Welcome to the K-World - or at least dabbling with the idea of getting an older K-bike. I was one of the few journalists that BMW invited to the World Press Introduction of the K-100, at La Napoule, France, in September 1983. I rode the first prototypes in France for just one day, and wrote that up as an article that appeared in the January, 1984 issue of Road Rider Magazine.
Later, I did a long-term test for Cycle News Magazine: over 10,000 miles. At the time, I was unimpressed by the K-100. Profound engine buzz through the handlebars and foot pegs, and overall lack of performance and power compared to other emerging Jap bikes at the time. At the time, I owned a 1984 Kawasaki Ninja 900, and a 1978 Ducati 900SS... so that is where my head was at, at the time. The Ducati was wildly uncomfortable to ride, but had character, and was just amazing in the corners. The Ninja? Stunning in all regards. Incredible power (for the time), smooth engine, great suspension, and effective anti-dive brakes. The K-100 offered none of that, yet at about 50% more in retail price.
In the Cycle News article, I panned the K-100. Why? All the above. This was supposed to be the 'next generation' BMW, yet offered none of the latest technology of the other brands of bikes, and had a footpeg buzz that would put your feet to sleep. And limited power. And on and on. BMW was very pissed off with that article, yet ultimately quoted me in a 2-page ad that they ran in all the cycle magazines: "In the words of Cycle News: A Gentleman's Motorcycle, just like the BMWs that preceded it."
Flash forward to now: 2 years ago, I happened across a 1985 K-100RT, full service records, mint condition. I remembered all the things the K-Bike did Right, and how it might be right for me, now at age 55. About 60K miles on the clock. I took the bike for a ride. Wow.
Modern brake pads, good tires, meticulous tune-up and maintenance, and... there was none of the vibration buzz that I remembered. Power was more than adequate, now that I was no longer a young canyon-carving Kamikaze in search of that ultra-last-bit of performance. I bought the bike.
What do I have now? When introduced as a 1985 model, I thought the K-100RT was german-frumpy in its styling. Yet in 27 years, it has aged very well, and looks contemporary to those who don't read cycle magazines every month. (Like when the Porsche 928 was first introduced, most hated the styling, but now it looks very modern.)
I have a 27 yr old motorcycle with full service records, and all faults are known (thanks to this forum). Except for bodywork, all parts are available as new (maybe through eBay), and most are on the shelf at my local dealer (unless I need something stupid like a piston). Yes, I still have some work to do to overcome some initial design faults on that first-year K-Bike (like gas tank gets too hot in summer weather).
My total investment? Less than $3,000 USD. And this, for a (I consider) a stone-reliable touring bike that me and the wife can take for a ride any time. IF there's a problem on the road, BMW dealers will look at this with a nod and a smile, and probably have the parts to fix it. That alone takes the value far beyond the purchase price.
So ask yourself: Classic styling that transcends time. Great reliability. Simple maintenance. Parts available everywhere, at reasonable prices (compared to modern bikes). Great support network, via this forum (Instant response from Inge, and great spares available from RJTrucker and others). Modern tires and brake pads available now, that truly show the capabilities of the original frame and suspension design.
If none of this matters to you... please go buy a brand new motorcycle that suits your needs. But don't be at all surprised when even the most basic parts are hard to find, or not available in 5 years.