Before writing any checks, take it to an inde transmission shop, pay for a pre-purchase assessment of the transmission. If you’ve never been to a transmission shop before, ask the shop where you get your cars fixed which inde transmission shop is the best in the area; i.e. which would they take their car to for a transmission problem?
He was probably telling the truth about being a flipper, but lying about being a mechanic
Any guy who has watched a youtube video showing you how to do a pad slap calls himself a mechanic nowadays 
Sure . . . he probably already did a tune up and realized the engine needs FAR more than that, to solve the problem. But he’s not telling you that part 
emphasized text when young and foolish I bought anything that ran and looked good ,if I got a year out of it I was happy. . now older and wiser I want 2 things to work on any car new or used and Thats steering and brakes