1. You want to take the rubber rim strips out and replace them with proper rim tape. The rubber migrates too easily, allowing the tubes to expand into the spoke holes and rupture.

2. The stock tubes are presta (skinny) valved. You can use the stock tubes in most new tires you might get, unless you get a really skinny tire you want to put on. The rim diameter of these wheels is 406mm. There are two kinds of "20 inch" wheels/tubes/tires. 406mm, like these, and 451 mm, which are larger. They are not generally compatible. Any new tube or tire you get should be 406. Schraeder (big, like a car tire) valves require a larger hole in the rim. The rims can be drilled out so a Schraeder valve fits and 406mm tubes with Schraeder valves are way easier to find than Presta. You don't have to replace the tires, but experience has been that the stock tires are heavy, squishy, and slow. As for me, changing out to a skinnier, smoother, higher-pressure tire was a revalation.

3. Seatpost- I still have the stock seatpost on mine, I've had any problems, and I can only imagine replacing it if a pretty white one became available at the right place. i'd highly recommned using the stock seatpost for awhile to dial in the heigh t and then making sure that the replacement post can get you the same height without violating the "minimum insertion mark."

4. Yep, I'd replace the saddle with something you like. I picked up my fancy "titanium" railed white one for 30 bucks on nashbar. Totally personal preference, get what's comfortable for you.