Another vote for using what you've got first. You may find after a match or two that you really dont like the sport. If that happens, you dont want to have sunk a lot of $$ into it unless you really dig it, which I bet you will.
The XD gear (holster and mag pouch) will suffice to get you started. You'll need at least 3 mags, regardless of which pistol you use. I started out using a Sig, then went to an XDm for a little over a year. Now I'm using a CZ 75 SA Target, and find it fits me nicely, and is more accurate than I am.