I am only a basic, average mechanic. I joined the NOC, bought and read Norman White's Commando resto book, lurked on this site for a month or more and read everything I could get my hands on. But I knew no one with an old Brit bike, let alone a Norton. I went to see 3 bikes before the one I bought and was disappointed with two of them (and one I took a dislike to the seller, who I didn't think I could believe). I ended up paying a little more than I'd originally hoped, for a bike that had been restored (by a firm that's still going) 3,000 miles earlier. A mk3. It had pages of receipts, a 40 page record of the worksheets for the dismantling and rebuild and 120 photos on a CD. All with frame / reg no. This was from 2014. Only 3,100 miles since. With further receipts for sprag and sprocket and carb service / choke replacement, plus 2 x services.
I spent a long time x-referencing the receipts with anything obvious I could check. They even let me open the timing cover to check the Boyer ignition, wiggle the wires, etc. I rode it 10 miles, but only after paying a deposit and leaving my car keys. All was fine. My concern was the bike had been hardly used for the past 5 years. As soon as I got it home, I checked it over carefully. Things I'd read about, that didn't involve dismantling the motor! Plus checked everything was tight and adjusted. All looked fine.
Despite this, I still had teething troubles. Wrong washers on the shocks meant the RH shock actually came off the bottom mount. One incorrect part in the hydraulic primary chain tensioner, caused the tensioner to seize pushing the chain onto the case. I found this when the sprag failed and the clutch became stiff (sludge in the basket). When the bike got hot, it leaked oil wherever it could. New gaskets, dowty washers, tighten downs and a reed breather valve.
About 100 miles in, the tickover became unsteady. The Amal carb needed a thorough clean out (+service kit), and a new rubber manifold adapter, due to a hole from previous over tightening of the jubilee clip, and the LH exhaust header had become loose. All needed sorting before it would idle properly.
All of these initial problems are now solved, but a further £1,000 in parts. I've now done 2,800 miles with my Commando. I am really delighted with the bike, it's everything I hoped it would be and more. But, I am hypersensitive to any noise, vibration etc.!
For me, the moral of my story is, researching and inspecting the bike can only go so far with an old bike. If you don't have the experience or skills to buy low / rebuild, there's a dilemma. Either don't take the risk, or buy, realising there's a risk and do what you can to manage it. The best thing I did was join this forum. I've loved learning and spannering. The silliest thing, was not leave myself with 33-50% of the purchase price, to fix what I found.