cc from what you've said, it sounds as though you have the desire and the finance to keep this cat but are mainly worried about
a) the risk to the cats you already have
b) his propensity to wander.
He's wandering because he's an unneutered tom. Unlike unneutered females, who just stay put and call, toms will travel miles to find 'love' from a female in heat (not easy to find in areas where cats are properly cared for and spayed). When neutered, he'd become a soppy lump of love, seeking affection from your family. Others with neutered toms will attest to this (Max, the only tom - neutered - I've had, was just a big teddy bear).
As you are concerned about the risk to your other cats, I would suggest that you have the virus test done BEFORE you have him neutered and decide to take him in. The next time he visits, lock him into a room where your other cats can't get to him. (FLV is only spread by direct body fluid contact, so you won't carry it on your clothes etc). Take him to the vet for the blood test - have the virus testing sample sent to Glasgow. And keep him while awaiting the results.
If the results are negative, let him join the household, and get him neutered. If they're positive, release him, feeding him when you see him.
Having the virus test done BEFORE you decide to keep him will be expensive, but will give you peace of mind.
Ela, and other rescue folk, may suggest that you get him neutered in either case, to prevent more kittens. You would get a voucher for this, or could hand him over to the CP if virus positive, they'll neuter him.
imo it would be nice for you to take him on, but put your mind at ease about his FLV status first.