There are 3 basic versions of the LT, with a few option packages to choose from. From '99-'01 (US model years) they bikes had ABS II (brake calipers say Brembo). From '02 to '04 they changed to Integral ABS (with a power brake pump, says BMW on the calipers). You could also choose from Standard, Icon, and Custom models, which added various options like heated seats and grips, 6-pack CD changer, on board trip computer, chrome package, etc. There was also an Elite/Exodus model that added an intercom, CB, and GPS.
The '05 through '07 bikes changed the steering geometry, added horsepower, changed the dash layout, separated the low & high beams into separate lights, and added the electro-hydraulic center stand. There were also some styling and trim color changes, and in '06 they added an HID low beam as an option. They also did away with different models and just priced the various options separately.
If you can afford it, the changes to the '05 and up bikes are worth the extra cash. The '02-'04 bikes are also great buys, and some good deals can be found if you look for them. But there are also lots of '99-'01 bikes out there with very happy owners. One reason people go for '03 and newer is a reduced chance of a rear drive bearing failure, but I wouldn't let that sway you from an earlier bike in good shape if the price was right.
So it all depends on your needs and wants versus your cash flow. But any LT that has been well maintained will give you miles of smiles.