Hi Mick,
Welcome !!
I don't have a B31 but I do have a 350 single (Velo Viper).. My A10 is also in bits at present but my limited riding of it so far would confirm it's probably what you're looking for..
The twin has loads more torque than a single yet at the same time it's a nice easy ride - the bike doesn't demand you go fast if you don't want to ..
I once had a Laverda 650 Sport IE , a twin,same capacity but much more modern, wouldn't tolerate top at less than 60 and the chain snatched at less than 3000rpm - an A10 is the exact opposite.
If possible keep the B31, it's always nice to have a single to use, less to go wrong too.. As for buying locally, I did that (Glastonbury, about 20 minutes away) as I was being lazy & the bike was also inexpensive as these things go. The downside is that although I rode it home and then around a bit I soon found it needed "the odd thing". It's still in bits now (I bought it last Autumn) so maybe it's worth travelling for a good one...!!
My choice was a late 1961 iron head swing arm A10 & that's one thing I wouldn't change..
Good luck..
Adrian