I agree with the canvas saddle pannier crowd. In that it's Spring time, you have time to shop. I frequently see used, serviceable saddle panniers advertised on Ebay, Craigslist, and such. I once bought a pair, and mended the worn spots with fishing line. You'll save a lot of money if you can good, used ones.
As for having the horses in camp, they need care and monitoring. You'll either need to pack in certified weed free hay, or have a campsite where the horses have adequate grass to graze. In my campsite selections, that can be easy or really difficult. I find places that have grass, but so many things that will snag and tangle a picket rope, you can't just walk away and leave a horse all tangled up. Depending on things, sometimes you need a fulltime babysitter with your horses when they're picketed/feeding. Take if from me, if a horse gets loose and is M.I.A., that is a horrible feeling to think you may never see it again. Our horses are like family members, and we lovingly care for them and like to spend our time with them. Beyond that, they'll add to the scent control issues you have to pay attention too.
Another option you may have is to thoroughly research where you want to go, look for outfitters in the area, and strike a deal for a drop camp. Then, you don't have all these responsibilities, and unforeseen problems and expenses. You pay the outfitter and you're done.