Check out the chest freezer specs here:
https://www.homebrewtalk.com/showthread.php?t=377518
Decide on ball- or pin-lock kegs and stick with one type. Ball-lock are narrower and taller (so fit better in a chest freezer with a collar) and have pressure relief valves, but they're more expensive. If you're brewing 15 gallons you'll need at least 3 kegs, but 6 will let you brew a second batch before finishing the first.
CO2 tank size depends on what is available at a decent price - keep an eye on the classifieds here and Craigslist - and how easy it's going to be to get refills. Having a couple of tanks is very useful; I run a 10lb tank to my keezer and a 5lb tank for general use (pushing starsan, purging kegs, on the road) and as backup. Always check and double check every connection for leaks.
Decide whether you want to be able to set kegs at different pressures (useful for different carbonation levels, or force-carbing one keg while serving another) in which case you'll need a multi-body regulator. If not, you'll just need an air distributor. Either way, be sure it includes check-valves!
Perlicks are the gold-standard for faucets, but there are plenty of other choices. Whatever the brand, forward-sealing stainless would be my recommendation.
Make sure your beer lines are long enough. Many places offer lines that are much too short - I'm running 10 feet at 10-15psi at around 38F.
As already noted, kegconnection is a great resource both for assembling a complete customized set-up
(eg. see
http://www.kegconnection.com/4-faucet-basic-homebrew-kegerator-kit/) and for adding extra pieces. They also offer - and honor - a significantly extended warranty and provide all-round excellent customer service.