Good thinking, Frank! It's good that you not only have a fairly firm idea of what you want, but you also are flexible enough to consider other options. Having had a chance to ride only one Airhead (the one I now own), .I'm hardly an authority on comparing your possibilities. But I have read and absorbed a lot of opinions, and I even remember a few of them.
Besides, a number of our members own more than one Airhead model, so I have no doubt that any misstatement I make will prompt someone to correct me (gently, I hope).
First, I'd like to add one thing to Monte's list of changes that occurred with the 1981 model: a combustion-chamber coating called Nik-a-Sil, but I'm certain of neither the name nor its spelling. Some experts believe that the coating solved a serious problem & thus is important. I wasn't around then, so I don't know.
One reason you haven't seen an R45 for sale is that, while they are fairly common in other countries, they are pretty rare in the USA. I don't think BMW Motorrad ever imported the R45 officially into the US. The few that we see here (and I haven't seen one) probably were brought in by returning riders residing (or vacationing) overseas. The R45 is a 450 cc. mounted on the same frame as the R65. I have read that the reason is that the German government has (had?) graduated displacement limits on bikes that new and inexperienced riders may ride. Learners and brand-newbies are limited to 450 cc.; slightly more experienced riders may move up to 650 cc. (There are power limits also for each level, but I can't remember them.) The result: an R45 is essentially an R65 with almost 30% less displacement and commensurately less horsepower. One of the reasons non-owners knock the R65 is its relatively low power and lack of top end. So, unless there is a super deal on an R45, an R65 in comparable condition should trump it.
Why did BMW introduce an "inexperienced-rider" BMW into the USA? The story goes that BMW wanted to introduce an "entry-level" or "starter" bike to attract American riders who could not afford other new BMW models. Once that decision was made, BMW apparently chose to import only the larger R65.
I understand your reluctance to go above 750 cc., for that was my limit for weight and size. Nonetheless, one peculiarity of BMW's model lineup in the 70s and early 80s might merit your consideration. Some of the earlier R80s, called "short-wheelbase (SWB) R80s have 800-cc. boxer engines mounted on what is essentially an R65 frame. Result: significant increases in power, speed and Insterstate-riding stability with no increase in size, little increase in weight, and little or no change to an R65's great handling. So, if an SWB R80 in good condition appears at a good price, don't summarily exclude it from consideration.
The question of /5 and /6 series becomes more complicated. I understand that the /5s (whether R50, R60 or R75) have a front suspension called an Earles Fork. There was a recent discussion of the Earles Fork on either this forum or the BMWMOA Forum (go to Airheads). A big advantage of the R65 is its very sturdy triple clamp connecting the forks to the head assembly, correcting a chronic fork-misalignment tendency of /5s. I recommend that you try to find and study the discussion if you're seriously considering a /5. (The other consideration regarding the /5 is market price; though not as high as the /2s, the price of /5s is increasing, seemingly at an accelerating rate. (I can't remember whether the /6 line changed the fork assembly.
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To complicate the analysis further, the R65 changed several times during its long production run. Other owners might classify more R65 versions, but I prefer three big classifications: (1) the small frame, dual-shock R65 (1979-1984); (2) the R65LS, which has its own fanatical following; and (3) the larger frame, single-shock (monoshock) from 1985-87. There's a recent critical analysis of the dual shock/monoshock handling characteristics on this forum, also. Monoshock R65s are relatively rare in the USA (fewer were imported), but you would be well-served to know what you would be buying, should one be available. Additionally, there were the smaller, but still significant, changes over the years. Example: a redesign of the R65's center stand in 1983, making the stand both easier to use and less treacherous when it is used.
In addition to this site's Trading Post, there's the BMWMOA web site's "Flea Market," which currently has 42 Airheads for sale. Guests may see the items available, but only members may access the details of a given bike. Suggestion: if you know a MOA member, get assistance in looking for a bike, then join MOA if a really nice bike hits the Flea Market. Also, local BMW clubs' sites (e.g., bmwbmw.org in the DC/VA/MD area) have trading posts that occasionally have good bikes.
Bottom line: you have a lot to think about, but it's fun, and there's a lot of advice and assistance available. Ask if you need help.
JT
