Hi Andy...Any WW2 military motorcycle will require a degree of TLC to keep it running well..It is the nature of 70 plus odd year old machines.
Looking after old bikes and learning how they work is best regarded as part of the pleasure of owning one, unless you have a local British bike shop and the funds to pay someone else to do the maintenance...
To minimise the amount of maintenance/repairs to be done the key thing is to buy one that has been rebuilt and well maintained...
That itself requires some knowledge. So, if you are unfamiliar with them try to find someone who is to help you make a good choice..
The products of any of the major manufacturers will do the job in more or less the same fashion...
Without doubt the BSA M20 survives in the greatest numbers and therefore gives you the greatest choice when searching for a good example..Spares are also pretty available...Matchless and Royal Enfield are equally well covered for spares...Both the Ariel and the Norton are good bikes though perhaps not quite so well supplied with spares...
I've had BSA M20s for decades and wouldn't hesitate to recommend one as a good bike that will give good service IF it has been well sorted and is in good condition...
Enthusiasts for the other makes I have mentioned could equally make the same statement though...
In conclusion my view would be not to worry which bike you buy from those mentioned...only that you get a good one!...Finally, get used to the idea that even with a really good bike some maintenance/repair will be needed over time...Ian
email (option): ian@wright52.plus.com