If you have to ask, you can't afford it...
Long waits to purchase any of the above aircraft new. Huge premium for used. Take your pick.
G500--Not as long legs as the 550.
G550--The king. See above re: wait time in decades (well, almost...). $55m basic.
7X--Not as long legged at the others. Nice though. Delivery positions can be had--for a price (I believe).
GLEX--Reliability issues. Long wait times (less than for a G, but still years away). Better cabin.
$65m for a used G550. I know of a couple coming on the market in '11.
Good luck. TC