(I just checked the posts after taking a really long time writing this before I post it, dinner and all. I'm gonna post it anyway.)
Welcome. These are generic questions, not specific to make, year, or whether it's a Combat or not. But they do beg to be asked. You said this is a one owner bike, so you're buying from the original owner? Have you asked him why he parked it for 30 years? Has he tried to start it? If not, why not? Any bike will sell for more if it runs, even badly. Will he let you try to start it if you change the oil, bring a battery, ect.? 90% complete, what makes up the other 10%? Depending on what that 10% is, it can be really pricey. And lastly, what's the gastank made of :?: :!: :!: :!:
Even if this was one of those 'Crate Bikes', which aren't just virginal, they are neonatal, there's issues on a bike that's been sitting that long. Anything rubber probably needs replacing. Iso's, tires, possibly seals, oil lines, seat cover, and even the wiring. The insulation gets brittle and the sheetmetal in the bullet connectors just crack all by themselves. Lastly, go over to BritBike and look under the Norton forum for a thread called Bad Shell Bearings- FROM LOOSENED CRANK SLUDGE. It takes a long time for that to happen, but 30 years would just about do it. And it bears repeating, does it happen to have an Interstate STEEL tank? If so it counts for a whole lot.