To extend on the answers here, more with an aim of showing what you'd need to do to alter different bits: (EDIT: Didn't see Locke's identical post before writing this
)
-Home Screen
-4 buttons at bottom
-7 Home Screens
These three are defined by the launcher, the buttons at the bottom is the dock, and the 7-home screens, note that the launcher doesn't massively influence what is on the homescreens it just provides the home screens, dock and apps drawer to put stuff on if that makes sense
-Music Player
-Hubs
-Gallery
-Camera
These are all either samsung apps (eg hubs) or samsung-modified android apps, the latter can be replaced, the hubs are essentially useless imo.
-Widgets
-Navigation
Widgets is a bit of a wide ranging one, many applications will install with widgets, or even be purely widget applications, but too generic to state further.
Navigation is stock android
If you root all apps can be uninstalled, not overly necessary, with regards to removing the icon then some launchers (like Launcher Pro) can hide programs from the apps draw.
Yep, android market is good, but also quite useful to get reccomendations from various sources such as forums and blog sites (the register for example have started a 'android app of the week' bit which is quite interesting.
As for price, some apps/widgets are free, others aren't, some have 'lite' versions for free and full-fat paid for versions...
Bit hard to give reccomendations but starting with the launcher there are quite a few options:
Launcher Pro (the 'plus'/paid-for version adds some nice widgets for texts/calendar/gmail that I love)
Go Launcher (seems quite popular, not really tried it properly)
ADW (again, some fans, did try it a while back and didn't like it, probably been updated many times since then so who knows)
There are also several others but those seem to be the top 3, it's always worth trying several different options to see what fits you best...