If you know someone who has a working 2005 GT for $1000, and it's an actual 2005 cab and not a 2002-04 cab that's been converted, you better jump all over it. Hell the monitor alone sells new for almost half that.
Retailers sell 2005's for almost twice that.
But....
You don't need to buy a 2005 cab. All GT's use the same cabinet starting in 2002 and just change the marquee and software, making it easy for operators to upgrade their games without buying new systems every year. The only cabinet differences are with the new GT Live's which change the hardware, CP, and add a LCD screen in a new header, replacing the dot matrix scrolling display.
If he wants it to play licensed GT software, then find a 2002 or 2003 that's cheap, use the extra money to buy the GT Complete upgrade, and play all the courses from 2002-2006. If he's planning on maming GT or something else in it, then just find a 2002 in good condition.