Find out what he towed with it. A light trailer shouldn't be an issue. Also figure on changing the transmission fluid. MINI automatics need to have that done to keep them happy. Not sure if a boost leak will cause any long term issues. You don't know if it it has a boost leak or something else causing the problem. Not sure I would believe the boost leak explanation for the half power mode it falls into. Have the codes read so you can no more of what's up. Is the seller the original owner?
My very best advice on this car is take it and have a pre-buy inspection done by ether a mini dealer or an independent shop that works on Minis. Two reasons one you will know exactly what you will have to have fixed and two you can use what you find to bargain the price down. Shops usually charge $100, sometimes they do it for free in hopes to get your business. If someone won’t let you take the car for a pre-buy WALK AWAY they are hiding something.