There are typically two straps on a pad. The one on the bottom, you slip the girth through and the ones on the top side will usually attach to the billet straps. It's used to keep the pad from slipping around while riding.
Usually I use a half pad on my horses but all of them have pretty prominent withers and I ride quite often and usually it's jumping. However, I depends on you want for the comfort level of your horse. I prefer half pads. What type of saddle pad do you have?
As far as it getting washed, it's going to get filthy after a few rides. It really depends on the weather. So for instance, my horses will sweat quite a bit during the summer so I'll wash them once a week. Just depends on when you want to clean it.
Again, depending on what type of riding you'd do, I'd more then likely go and get some half chaps with some paddock boots if you are planning to do just pleasure. I prefer Ariats because they last forever, as far as half chaps, just hop over to a tack shop. You can usually try on everything there, so instead of measuring and ordering, you can get a feel for it on you instead of guessing. I wouldn't recommenced tall boots unless you plan on showing because at times they get a bit expensive depending on what you look at.