I feel for the OP here.
For me the contract is the least worst option, assuming her actually wants to help out Step-Son.
There’s not info to suggest Lad is a bad payer, just that they’ve fallen out before.
Call it an agreement, call it a contract whatever is more palatable, it’s unenforceable really – OP isn’t a licensed credit broker or finance co. I suggest ownerships transfers at the 50% mark – that’s how it works with Banks. V5 after all is a record of keeper, not proof of ownership, OP is more liable to keep it in his name.
This could be a great way to re-enforce your relationship. Be honest, acknowledge you’ve fallen out before, explain agreement just there so there’s no misunderstanding, ask if that’s okay – if it’s not then you’d rather not do it. It’s fair – it might sting for a little bit at first for him, but better now than later when it’s a real problem. Shake hands on it.
The alternative is turn him down, that will cause bad feeling, or don’t have an agreement in place and hope neither side does something they think is totally acceptable and fall out over it. One of my mates did a similar thing with his Dad about a Motorbike caused two huge arguments – Mate was a laid-back type, always paid, but would wait until he was going around and pay in cash – that might be the first, or even second week of the month – Dad expected money, in the bank on the first of the month – as a bank would – but didn’t say anything until it was a payment number 6 and it was a huge stress point for him – also, Mate sold the bike after a year, still owed his dad a couple of hundred quid (it was £4k to start I think) Dad expected to be settled when he sold it, Mate wanted to use the money against new bike and carry on as they were – again, two months later huge argument. Get it agreed upfront!