All sounds a bit over complicated to me.
I think in general lease companies prefer not to deal with these sorts of enquiries, not saying OP is but they must get loads of time wasters/chancers etc and no doubt people moaning about stuff too.
Auction route is the easy option, zero comeback and hassle free.
I’m a bit suspect of those prices if I’m honest, but private buyers can’t expect to purchase vehicles at trade prices anyway.
But if OP really, really wants this specific van then put the effort in and play the game. If you can’t get it for what you want from the lease company find where it’s going and get someone to bid on it for you.
For a £3k saving it would be worth the effort.
An MOT is £40? If it fails you know what’s wrong with it (and maybe wouldn’t buy it if so or it will sell for less at auction)
Just a thought