Modded car with a boost gauge at a dealership... in my experience this SCREAMS problem child but it might not be. If you are experienced with these cars and know what to look for it might be ok.
As for that 6 speed. Its likely out of an 03/04 Cobra which had a VERY low 1st gear at 2.66 so 4.10 or even 4.30s are kind of required for a NA 4.6L running one. They have a .62 final drive to keep the RPM down on the highway but the real purpose of the 6 speed is not for some ungodly low overdrive, its the 5th gear in between 4th and 6th.
At any rate, go take a look at it but I'd be very very wary. IMO, it was put "back to stock" or damn near by the previous owner but it could possibly be a car that was being set up for boost and the guy ran out of money and sold it to the dealer. You never know.
edit: And are you sure it was "boost gauges"? Lots of ppl, myself included, put aftermarket oil and water gauges in these cars because the stock gauges are horrible.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.