I think you are right.
There are scare mongers on here, but IMHO what the seller is doing is perfectly normal in a private sale.
The only issue is the size of the deposit - £100 should be plenty for him to hold the car for a week.
at the end of the day, all he should be looking to cover is his advertising costs, should you bail out.
Your plan to get him to pay off the finance at the same time as you give him the money is perfectly normal.
I did it once when buying a Colt CZT (dont ask

) The seller rang the finance company on the day, got a faxed settlement figure and then I went with him to the bank and paid the money off his finance.
Funnily, the car (2005) was 18mths old. It cost £15k new. I paid him £4500 and he then had to find another £4k to pay off the £8500 finance that was outstanding!! The car was a pocket rocket which gave me 6 months of fun. I then sold it for £5k to some mug who drove all the way to me in Aberdeen from Birmingham!!
Bottom line is, you clearly know what you are doing with this deal, and if the car is right for you, you can make it work. If you have any doubts, walk away. As I said earlier, apart from the big deposit, I dont think the seller is being unreasonable.
Andy