3) Is it true that the software can be easily updated? So if I bout a Golden Tee 2006, I could update the software easily?
There is no Golden Tee 2006. In 2006, they released the Golden Tee Complete, which was the last of the GT Fore! systems. GT Complete contains all of the courses from the 2002-2005 releases, and is probably your best bet - if you can find one (I think there is/was one for sale here in the For Sale section".
After 2006, they change to the GT Live! setup, which is completely different than the Fore! setups. Basically, it now has a PC inside it with an external video conversion PCB for use with non-VGA monitors. The conversion kits for changing Fore! systems to Live! systems also included a small LCD screen that is mounted up top in place of the old LED banner. The conversion is fairly involved, and is not the easiest thing to do for someone with little arcade experience. Twice I had to finish a conversion for someone who started it, got stuck, then when they thought they had it done - it didn't work.
You can upgrade the FORE! systems easily by replacing the PCB and matching hard drive.
You can buy any GT from 2002-2005 and upgrade/downgrade it to one of the other years (02-05) by swapping the security chip and hard drive. GT Complete upgrade kits were released for a while, but the upgrade required a call-in to ITS Games for authorization, and I don't think they support it any longer, so you most likely need to buy an already-installed setup, or a complete working game.
Unless your boss is completely set on GT, you can probably pick up a nice PGA Tour Challenge or PGA Tour Championship version for less money. It plays the same, usually has more courses per machine, and has some other options that GT doesn't have (like topspin, players who are left-handed or female, etc)...