I would go with a brown Wintec AP saddle. The Wintecs have changable gullets, so you can make sure you get the right fit. In a Wintec, he would likely be a medium-wide gullet.
A brown saddle will have a higher resale value and an easier time selling it should you decide English riding isn't your thing, or you decide to show more or switch to Dressage.
If you don't want a synthetic saddle, then a good medium tree All Purpose saddle would likely fit, like an older Stubben, Passier, Courbette, or Collegiate. Older Crosby medium trees may fit as well. If the tree comes in cm sizes, look for a 31-32 cm tree.
English saddles run differently, so a medium in one brand may fit but a medium in another may not. The size can alsy vary among the models within a brand, or new vs. older modles. Most stock horses need an English saddle with shorter tree points, to allow those big shoulders room to move. A medium tree with short tree points will fit better than a medium tree with longer tree points.
If you want a fixed tree saddle, you need to work with a local shop or an online that does saddle trials. A good online shop can also have you take wither tracings and narrow down your search based on that and photos of your horse.
For your size, you'll most likely need an 18" seat, but it depends on how you fit in your current 16" saddle. If you have some room in front of your thigh (3-4") when you're seated in the middle of the saddle (not up against the back/cantle), then you might be fine in a 17.5" English saddle. If you prefer a snug fitting saddle and only have 1-2" of room in front of your thigh and like to sit more against the back of your saddle, then you may need an 18-19" English saddle.
Again, working with a shop would be helpful. A good online shop can look at picture of you seated in your current saddle plus your height/weight and they can help you find the right size. If you have a local shop where you can sit in saddles, you want to have 3-4 fingers of space behind your rear end and between you and top of the rise of the pommel. You want room to be able to post the trot comfortably without going over the front of the pommel.
Do not, I repeat, do NOT buy a cheap English "starter package". They are not made well, have ZERO resale value (littleraly, none), rarely fit horses well, and rarely put the rider in a decent position. The best way to use your money on an English saddle is to buy a used European or Argentinian made saddle, or a good synthetic like Wintec or Thorowgood. If you stick with one of these options, you'll be much happier in the long run, as will your horse