personally i would rather buy a car from a forum member than off a dealer....when you buy off a forum member you know you are getting a well looked after car, most cases you can view the members posts to see the history off the car and how well it it looked after.
buy from a dealer and the history of the car is a grey area, you get a salesman who probably knows very little about the car telling you its very well looked after even though he most likely has no idea of the history, they will tell you anything to get the car sold.
its your choice at the end of the day but i know what i would prefer
as for the remap..just because it has been remapped doesnt mean the car has been thrashed. i got mines mapped purely because the car performs miles better, easier to overtake, more responsive and you dont need to rev the tits of the engine build up speed