Didn't have time to read everything, but we have ~8 acres of pasture and 2 of "yard." We had a small JD tractor with pull behind mower and bush hog. We sold it all and bought a 70-inch commercial zero turn mower (in EUC for ~$6,000). Best thing we ever did. We mow the fields and the yard, and it takes half the time of the tractor! It's awesome. That said, I don't have manure to get rid of (horses never stay in stalls). We drag the fields with the mower.
Just an update: I decided to buy a TC45DA tractor with 16LA loader c/w skid steer attached bucket. We'll use it to drag arenas, move brush, mulch, and manure piles with the loader, and in winter it should handle snow removal. We already used it to pulling arborvitae that needed removed using a chain hooked on the drawbar. I'll be buying an SSQA bale spear and SSQA pallet fork attachment to handle moving large square bales and pallets, respectively.
For mowing I chose a 61" Ferris IS3100z zero turn mower. It is extremely fast (wind in your face fast) and also comfortable to ride. You can mow pastures with it if you don't try to fly along. I could have sprung for a 72" model but I don't think it would have fit very well between trees and other obstacles around the property.
I also bought an 18' car hauler for moving the stuff around. I don't expect to do this often but I kind of wanted one anyway and it is already quite handy. I'll be picking up a few pallets of bagged shavings on Friday too.
Sharian makes a great point recommending a Skidsteer. Consider your needs as follows:
Tractor- as you descirbed but reconsider the 4WD. It is overrated and you will PAY for it. You said you had flat ground--you won't need 4WD.
Skidsteer--nice small foot print and easy to turn and you can get a scoop for it to turn your manure pile, move earth etc. You want a scoop on either this or your tractor
Bushhog--with 15 acres you will need this. You can get by with a single wheel (vice double wheel) in the back. A little less expensive than the dual weheel. Your tractor will pull this nicely.
John Deere Gator or similar vehicle. We have the 6 wheel with 4 wheel drive( 4 wheel not needed). This is great for daily manure pick up and haul to the manure pile. Wheelbarreling to the manure pile is tedious. Also can use to drag your arena; very manuevarable.
Manure Spreader--you did not mention this but you will eventually need it. Use your tractor to pull and you will want PTO vice wheel driven.
Take away--the biggest workload associated with horses is the manure handling. As they are large producers of it, you will want to consider equipment that you will use daily to handle it.
We've considered adding a Gator but they are quite expensive for what is really a runabout device. A dump bed golf cart might be more our speed and half the price. It would save the wheelbarrow trip, but in fact right now with 5 horses and a typical day inside, we fill the wheelbarrow only once. We've found it helps to use shavings that sift well even if they are expensive - because you use less of them and get done quicker.
We don't need a manure spreader because 1) there is nowhere to spread it and 2) there is a local pig farmer who drops off his spreader when we call him and he'll take it for free as long as we load the spreader.
We've used the tractor's 4wd once so far pulling arborvitae. In winter it may prove useful again. (Just about every tractor I considered was 4wd). The tractor does a lovely job dragging the arena and the loader bucket has been used regularly for re-piling manure, moving stonedust, mulch, and hay bales.
Haven't needed a bushhog since the mower is such a monster and everything that needs mowed goes no more than 3 weeks. But I might begin mowing the neighboring cornfield perimeter in exchange for being able to ride said perimeter, and it may be a bit rough for the mower. We'll see.
Right now the big addition my wife wants is a mechanical bedding sifter. Doing stalls daily has inflamed her carpal tunnels and long term it doesn't appear sustainable. My father-in-law bought one for his wife a few years ago (she runs a boarding barn) and she loves it. They brought it over today and tried it in our stalls and it works really well and saves a lot of wear and tear on the forearms. Just shovel the contents of the whole stall onto it, dump the muck bucket, and spread the shavings again. They are expensive though and nobody ever sells them used, ever. If you're keeping track we have funny ideas for July 4th celebrations and Christmas gifts.