If it's a V50, not a V50 II, run away! Chrome bores… II is nigusil, Guzzi's nikasil. 78 might be a II, I'm not sure when the model change was off the top of my head.
Speaking of heads, 20k miles is around the time when things in there can get ugly. Perhaps materials were better in the II (I'm hoping so as I have one) but the exhaust valves in my Monza, and others I know of, started stretching around 22-24k miles and were replaced. Around $600 for that and helicoiling everything that needed that (most threads) from Edco in Petaluma. Better than new now. The other ugly thing around that mileage is the rear drive. Many threads on that -- the pinion bearings get little oil in stock form. There's a fix, DIY if you're Chuck, send it to Sean Fader if you're me. And say bye bye to another $700 or so in parts and labor as I found mine (again in the Monza) was bad when little silvery magnetic chunks came out in the oil. If you get to it before the bearing are toast you save a lot! My V50II has around 16k miles so far with no problems. So far…
V50 and V50II are geared pretty low and are lotsa fun on backroads. I'm not sure where you are, maybe Marin or Sonoma counties? Great habitat there. Freeway is another story. Topo Gigo will cruise happily at 100 all day long as long as that's 100 kph. Keeping under 6000 rpm for cruising means keeping under 65 with bursts to 75. Life in the slow lane in CA. Monza is geared higher and the III engine has more mousepower so it will happily peg its 80-mph speedometer.
There was a V50II for sale at the recent San Jose Euro/Japanese bike show. Looked like a possible parts bike until I looked at the seriously cracked rear drive housing… you pay me to take that, pal. Or, "is that the San Andreas Fault?"