The mk III is the best of the bunch, so you've got the right model. 800 on the clock is crazy low. These bikes normally have high mileage as they were workhorses and never collectors items really. So it sounds like you got a good find there.
Mint - $ 3000.00 to $ 3600.00
Good - $ 2000.00 to $ 2500.00
Ratty - $ 800.00 to $ 1500.00
Basket case - $ 500.00
I paid $ 1700.00 for my '79 MK II.
I've pretty much had to rebuild every single mechanical component on it.
Which has cost me way more than if I had bought a mint bike.
Guzzi parts can be costly, so a good bike, like the one it seems you've bought / thinking of buying is the way to go.
They're great little bikes btw.
What do you plan on doing with it?
And, yes, pics please!
Dale