In this thread are good hints on starting your rack, especially CBS's links to two other threads on the topic. One quick way to pick up some biners is to buy a six pack of quick draws. You'll use those cragging, but if you go alpine, you can take the biners off and add those to some shoulder length tied slings of 9/16" climb spec webbing for more versatility.
Other good advice, start off slow. You don't need a full Yosemite rack, just a set of nuts, the larger half of a hex set, some racking biners, and maybe an abbreviated set of cams from 0.5 - 2.0 if you have the spare cash, but I'd hold off on those till you need them.
If you're cragging, you won't need to worry about water purification. You'll be able to carry the water you need. Aqua Mira (chlorine dioxide) is a lot lighter than a filter. A filter, stove and tent are group gear, and you can probably get by with one of the three and partner up for the rest.
Despite the online personalities, most of the folks here are good people. Have fun.