Hi Chris
The burning of exhaust valves is caused by running the bike for long distances with the ignition retarded, if the ignition timing is set correctly and the advance/retard lever is used correctly then no problem will be found with overheating, also be aware that this applies to any old bike weather overhead or sidvalve, the problem is caused by either ignorance or carelessness.
As for which bike you decide to buy, it has to be a personal choice, both the Norton and the Ariel are good bikes but to my mind possibly over priced at the prices stated, there is no real advantage buying from a dealer as non that I know of will give a guarantee on a bike of this age, my advise would be to join the VMCC and attend the local meetings, these people are the real enthusiasts and can often point you towards members and friends of members with bikes for sale, take your time and find the bike you really want buying privately from a genuine enthusiast will produce a far better/cheaper bike than you get from a dealer.
Also remember that buying the right bike at the right price is not money spent it is money invested, buying from a dealer you have lost money as soon as you pay for it, you may well find that over time as the bike appreciates in value you make up the loss, but better to buy right in the first place.
Most enthusiasts are proud of their bikes and would baulk at the thought of selling a duff bike to a fellow enthusiast, they are more likely to get rid to a dealer!.
Regards
Dennis