Go with dark brown leathers. You'll want them a bit long if you're tall (5'7" or taller, or have long legs). If you of average height, then regular length should be fine. I like jointed irons if you can find some reasonably priced. They make it easier on your knees/ankes, and easier to stretch your legs down long.
Does it have short or long billets? Most Tristans I have run into have short billets. If so, the same girth you use for a jumping or hacking saddle should be fine.