Yikes, I ride an '84 R65, and like you, returned to motorcycling after a looooong absence about 2 years ago. I'm also a big fan of the R80, and actually began my search hunting for one. I'd still love to have one someday.
That said, you'll find the R65 is lower, a tad lighter, handles quicker, making it a much better around-town and short-trip bike. The center of gravity is perfect, and with the quicker steering geometry the bike goes wherever you "think" it to. It's also geared lower and revs quicker, being a short stroke engine, so it'll run neck & neck with the R80 and, really, the R100 up thru about 3rd gear, when their taller gearing and greater torque kicks in. I know I've "dragged" a friend with a very strong R100CS, and I've got him up to about 50-55 mph.
The downside, of course, is that you start picking up vibration right about 65 mph, which does go away again around 75-80, but your rpms are also pretty high by that point. I've ridden my bike at 80-85 for 25-30 miles at a stretch and nothing "blew up", but I wasn't the happiest camper and neither was the R65. It'll do it, though, probably all day long, day after day, just not very comfortably, in my opinion: others may disagree.
The R80 has the very same 50 hp, about 7-8 lbs more torque, and , most importantly, taller gearing for lower engine speeds and a calmer ride at higher mph. It's also slower revving, has lazier steering geometry (slower around town, but more stable at speed), and handles about like its bigger R100 brother, with which it shares lots of parts, which makes finding used stuff pretty routine.
All depends on where, how far, and how fast you're gonna ride. If, like me, as a returning rider you plan to do mostly local and short-trip work, especially on the backroads with 50-60 mph sweepers, the R65's a true smile-producing machine! If you're more into the Interstates and cruising at 75-80, then the R80 might be the bike you need.
But, they're all good, so you really can't make too much of a mistake, and unless you grossly overpay, you can always resell and get your investment back, at a minimum!
Good luck, and welcome! [smiley=thumbsup.gif]