Hi Anthony, welcome to the site! You might want to consider adopting RES, they are not hard to find and should cost nothing. You might even snag a complete (though inadequate) setup to get you started.
40 or 55s go on sale often at the $1 per 1 gallon rate. You might want to keep an eye on used tanks -- 75, 90, 100 gallons also go cheaply used, sometimes with stands included.
Skip most equipment with the word "Turtle" in it. Fluval, Eheim are good brands. Spend a little extra on one the heaters Kansasslider mentions.
You want lighting to be over the basking area, so a combo will do. Separate fixtures are good too and worth considering, but that depends on the layout around the tank.
I'm not a fan of floating docks. Acrylic, resin, pvc/tile, cork or ATBA are good ways to go.
I don't keep plants, you you might want setup a small planted tank to use for your RES.
Most timers will work for the lights, even cheap ones. Just get a 24 hr one. Forget Digital thermometers... get a LCD stick on and an infrared temp gun. Moss balls won't do much for you.
1) Should I go with sand or large river rocks (or bare bottom)?
If I had a choice, I'd do sand and a couple of large and smooth rocks for color.
2) Will that filter be ok or should I go with something bigger/different brand?
Go larger.
3) What else am I missing?
Stand... don't forget supplies to help you with water changes. Get some bulk carbon for your filter.
4) If I wanted to get 2 turtles I'm assuming I'd need 100+ gallon tank?
Sure, but you might need to 2 tanks if they don't get a long.
5) Anyone in the Westchester NY area selling anything I could use? (if you don't ask the answer is always no
)
Craigslist is your friend... also check out reptile expo on
Sunday, November 29
10am to 4pm
Westchester County Center
198 Central Ave
White Plains, NY 10606
Good luck! If you want to adopt nearby, check out turtle rescues LI.