Got my eye on an 84 R80st, anybody got any experience with one of these?
In a word bulletproof. Has the same monolever as the G/S but with a shorter shock so it doesn't articulate it anywhere as much as the G/S - the instruments should look familar to you :-) The forks are an improved version of the R65 forks and not a shortened version of the G/S forks.
The high exhaust means that you need period G/s luggage (if that is your thing) with the smaller LHS pannier.
The R80 engine is the pick of the airheads for smoothness and the extra power (over say the R65) is nice to have whilst getting pretty much the same mileage as an R65.
Engine wise the first model ST came out in 1982 so it has the potential to be caught up in BMW's unleaded fuel debacle - although to be honest those problems should be long sorted by now. The model year-span also can include the dread gearbox circlip problem - but again there are circlip free boxes out there that have done hundreds of thousands of kilometers trouble free (it is just that if that one bites you the new input shaft is an eye-watering price.
All in all a good solid model - albeit that as a "bitzer" things ca get awkward if you need to replace things like say the rear mudguard, side covers etc (and you already know what an R65 instrument surround costs).
I could have bought one of these a couple of years ago with a blown gearbox and an obviously tired engine. At the time I didn't want another project as project to stand the R65 back up was busy costing me more than twice my original estimate. I passed but have regretted it ever since - the rareity of the model will mean that eventually they will command a slightly higher price than the "cooking model" R80, but you will probably have to wait a while.