The higher power ( whichever flavor you prefer ) of the universe has smiled
upon you by causing the events that led to the owner showing his bike to you.
This is how many of the very best things in life happen. You are not looking for
whatever it is, and yet there it is, dropped right in your lap.
You could search for months and not find a clean original bike like this one.
There is a rational way to arrive at a value for this bike. Price what a superb example
is worth, and then price what the best BMW mechanic you can find will charge for
going through the entire bike and putting everything in order. An experienced BMW tech
will be able to tell you what is likely to be wrong. You might find that tires, a battery,
a carb cleaning, front brake system rebuild, and maybe a few seals are all the bike needs to run initially,
and after that you can sort out the finer points. It would be wise to add a "fudge factor" of $1000 or so
for the costs of problems which may appear once the bike begins to be ridden.
In any case, if you want the bike don't delay because such deals sometimes can slip through your fingers
and bikes like this one do not come along every day.
PS - I bet there is more to the owner's story than a stuck throttle. Maybe you can
get a beer or two into him and he will tell you the rest