Hi George.
Were not Haters, but we do get are share of trolls here trolling to start sh!t from time to time. I read your first post ad waited to reply to see if you were being serious and a troll, I feel that you are being serious so I will treat you as you are asking to learn.
In touring models (And the frames are all the same between the sub models) The twin cam frame started in 1999. The early ones had issues with the first cam design (The gear would strip the wood drift key). They changed to a spline in 2000 but the engine still has cam issues. Most of this was found to be oil galley issue with dirt from the 30micron filters Harley was using, they now use a 5micron filter. There was still and issue with the can spring tensioners, they would fail without warning. S&S came up with a gear drive cam system to eliminate the chains. In 2003, Harley changed the bottom end going from the "Timkin" three bearing flywheel set up to a two roller bearing set up. There both good solid set-ups, but the roller gave more movement to the crank and this made installing the gear set up more problematical, as the run out could chip the gear teeth. Harley fixed the chain issue in 2007 with the introduction of the latest twin cam the 96. The new cam chain tensoinors used oil pressure instead of springs and used a better nylon pad that did not wear out.
Now to your question about 2000-2004. Up until 2002 Harley used the Marrelli injection system, while it worked fine when it works, it is hard to fix and expensive to get parts for, many of these bikes have been converted to Carburetor. In 2002 Harley changed to the GM/Delphi system, and it is still in use today and it far easier to work on. 2006 was the last year you could get a Carburetor. So if your looking at a 2000-2001 be sure it has the carburetor.
Any of them you buy, be ready to change the cam system over to the new Harley system, you will need the cams, chain plat oil pump and so on, Harley sells it in a kit. (Assuming it has not been changed over). Be ready to install a true track (Again, assuming it does not have one) all the Harley tour bikes have an issue with tracking and this will correct it. I would also suggest having the front motor mount replaced, they usually wear enough in 15,000 miles to warrant replacement.
Try to avoid used bikes that have been heavily modified, it usually is a nightmare. The more stock the better.