There is a lot of knowledge on this forum and many helpful people.
A few comments:
Commandos - especially those sold east of the Mississippi were often sold with "undated" titles as sort of a scam. There was a significant break ("brake?") between specifications starting at New Years of 1972. You can identify a motorcycle with the factory 1971 spec by looking at the engine case next to the serial number -- if there's a metal "L" fitting and a rubber breather hose, it's factory-spec for '71 and you should always buy 1971 parts (if you decide to pick this motorcycle up). If this breather isn't there, it's a factory-'72, parts accordingly.
The H*-R*d*r (the motorcycle sin that must never speak its name) was introduced in mid-71 as the same time as the "SS" (street-scrambler). I don't know if it was meant to be a short-term model* but it managed to last while the "SS" quickly faded away (Berliner refused to take the "SS" models after a while). The tank on these models was modeled on the tank on the Norton P-11 desert racer of the mid-late 60s for the "SS" and carried over to the HR. They were glassfibre until very late in the Commando's life when a few HRs ('74 and '75) were built with steel tanks; the steel tanks will replace a glass tank (and should) but they're hens' teeth and expensive. The first HRs were built with drum brakes and that was continued into 1972, even when the Roadster and Interstates were supplied ~100% with discs but about mid-72 (or maybe a bit later), the disc became standard on the HR, too. (I personally wouldn't use a 50-year-old glass tank for anything but cutting the top off of it and using it as a houseplant pot but if you happen to get a very good one and you have access to non-ethanol fuel, one might work for you for a while -- still, I think you're going to need to plan for a new steel tank. I take that back -- my original Production Racer glass tank from 1972 is going to hold my ashes in the not-too-distant future -- but you get my drift here.)
If you like a drum brake, there are some "improvement" kits that, added to careful assembly, will give you a very useful and enjoyable front drum. (Lipstick on a pig, maybe, but if you want a drum, it's worth doing.)
You say that the bike that you're looking at has "extended front forks". If you intend to get any of the performance that these motorcycles were built for, you'll need to get rid of those. They can make for some really nasty handling/stability situations. (I have some fork legs - stock - with bushes and seals that I'll send you for free if you pay the postage. They're a little pitted in some places but they'll work OK for a while. They come with the bushes and seals because I'm redoing my front forks with Don Pender's "Lansdowne" dampers and NYC Norton/Constantino polymer bushes.) It would be a shame to put work and money into a Commando that you could only putter around on like an old lady without fearing it was going to throw you on the road and stomp you butt - AMHIK.
I guess you've gotten the idea that I am not crazy with how this particular motorcycle sounds as the subject for a project to become an enjoyable rider. You seem to have a great attitude and the background to do a good job with anything as a "starter kit" but if I were you (especially considering the rust and corrosion issues on this one), I'd look for something in better condition.
Also, don't pay too much. Back in the day, this model was highly prized by a few people and they were keen to pay a premium to obtain one but the overall market value wasn't impressive. There's a mythology that they're a rare and particularly valuable model -- which is true for "SOME PEOPLE" but the average rider isn't likely to pay much more than for a similar Roadster or Interstate model. If the seller thinks he has something $$$pecial and wants a higher than the going price, it might be a good thing to make polite apologies and just back away.
