Free: Contests & Raffles.
You are off to a great start. Go to a bike shop, get measured, and see how the people there fit you for a bike and listen to their advice. Then go shopping on craigslist, where you can save $$ over new. As a general rule, if your body size/type match the seller/user, you should be off to a good start. Note things that can be changed are handlebars (height, sweep), including gooseneck, which can change how far your body is hunched over (don't want too much, too much hand pressure and back strain) and saddle type and position (a slight amount). So you'll want to get in the ballpark with the basics of frame dimensions and geometry, because you cannot change those after the fact. Then take your knowledge from the bike shop and test ride the craiglist bikes. Another option is pawn shops. Do buy all your consumables and tools from the bike shop though, at least, to reward them for schooling you on fitment.29"ers are all the rage with bike people, but don't rule out a 26"er if price and fitment are right.Extra tubes, a patch kit, tire tools, a basic bike oriented multi-tool, and a pump are about the only essentials. You could pack a spoke wrench and a chain tool, but they are probably not essentials. If you are planning on muddy and wet terrain, plastic fenders might be nice, and a rear rack will enable you to hold a limited amount of gear.
Thanks for your service, BTW.
Tagging along One thing I might do that I've seen, is get a 2nd hand child's cart to toe behind for gear/game