First, welcome. You will find good information here.
Wheels are an easy swap. The '03 and newer are 17" and you can find them in regular silver, hypersilver, and chrome on
ebay. Regular silver will be the color of yours. Hypersilver has a little black cast to it. Chrome looks nice but is pricey. Your old rims will be of little value to resell, so you might want to save them for snow tires if you live somewhere cold.
You will want aftermarket DVD. Factory has a small screen and would not be cost effective to install in a Seq. that didn't come with one. Rosen, Alpine, and Directed make good units with 9" to 12" screens. Some 15" and 17" screens are out there and might sound cool, but they get in the way and aren't that easy to see in practice. You also have the option of headrest monitors if you like that kind of thing. Most DVD units come with headphones, but we usually listen to ours over the factory radio. The usual way to do that is through FM modulation. That is adequate, but has static and radio interference problems. Better is to buy a USA Spec iPod interface. It has a separate jack that you hook your DVD sound into. It sounds great -- CD quality sound. It also charges and plays your iPod through your factory radio.