TBH the garage is probably going to be loyal to the customer who gave them all the cash for a rebuild so they would say its good.
But if they seem a respectable company you have no reason to think the work isn't good.
I wouldn't worry about stamps in the book if he has receipts etc. Lumpy idle sounds normal ignore that.
I hate buying a new car because I'd get too excited that I want that car and not make sure its the best possible deal I could get. What you need to think is whats it worth and could you get a cheaper, better looked after car elsewhere.
Whats his reason for selling it straight after a rebuild?
Remember he's trying to sell it and he probably doesn't have many buyers as they will be put off by the warning light and high mileage. If I was you and had checked out the car properly and it was he says it is. And the warning light will be a simple fix i'd offer him what a standard one would go for. Maybe 9k and then just don't seem interested if he says no at first.
He's probably struggling to sell it.