Surprisingly cheap to insure. My lad at 21 a few months after passing his test was insuring a 900cc bike for £300ish. Much cheaper than his car. I pay less than a tank of fuel for my insurance.
Depends what you want the bike to do for you, but something like a c90 should be easy. You can pick up engines for £100 and there’s all of 2 bolts and a wire to deal with to slot a new one in so no more difficult to maintain with than a bicycle. Easier if anything.
As they get bigger they get more expensive and complex but something with one cylinder and under 500cc could be possible to buy for £500 and not require £1000 worth of tools to keep running. If a c90 isn’t up to the job something in the 175-400cc range probably offers the best value as most people want bigger or smaller, but still cheap and simple to maintain.
EDIT: missed the salisbury plain thing. Something like an XL185, 200 or 250 would fit the bill.