That is really nice that you are doing all that.
I'd take anything out of the pasture area that I could. Horses have a way of killing themselves on the smallest thing. I don't cut anything off at ground level, and backfill and compact the area after I took out the junk.
And of course some types of trees, you don't want around horses - any of the cherries, black walnut, a lot of the ornamentals, ewe shrubs, etc. You can get a list from your ag extension office or look it up on the internet.
It's better if it comes right out.
It won't be at ground level for long if you leave it there. It will continue to grow, the ground will erode, etc, and in no time it will be an obvious hazard. Too a horse has a flat large foot and what doesn't seem like much of a problem to a human or dog or smaller stock is a problem with a horse.
You might especially take a look at trees that will be inside, but near the fence. You'd be amazed at what a horse can make happen with those. I'd leave 20' between fence and any tree, more if I had over 1 horse per acre in there.
We took out all the rocks, trees, saplings, the whole nine yards.
Where we had a really nice tree we wanted to preserve, we either had it outside the fence or so far in from the fence, that it wasn't a hazard.
And of course the other reason is that when you have to mow, you don't want to damage the blade on your tractor or mower. Honestly it is awful trying to work around a lot of junk no matter what you're doing - seeding, reseeding, fertilizing, liming, mowing, fixing fence...all those little saplings and junk become a huge pain in the backside over the years.
Of course I also like that I can mow everything with a bigger machine and I don't have to go out a second and third time with a little mower, a weedwacker and fuss a lot to get things mowed.
I even like having a clear strip on the outside side of the fence - partly because I have electric and that helps to keep it working and makes maintenance easier, but also because I can control poison ivy and other stuff that's trying to make its way into my property, by maintaining that outside strip.
Oh yes, mowing. That's another part of maintaining a grass pasture. All livestock graze unevenly - if it's left to the animals to do the mowing, the pasture will be full of weeds and woody shrubs in a year or two.
As far as a straight fescue mix, I wouldn't. Because of the issues with fescue itself, because a good many horses don't find it all that palatable, because under some growing conditions it grows too fine for horses (and causes impaction in the gut) but also because no pasture of one type of plant, is going to succeed.
It's better to buy a standard mix that's designed for your area. What plants are in the mix and at what percentage, will be different for each region.
That way, the plants that are more suited to your land will do well - also with a mix, it's set up so there are early season and late season grasses - when one is affected by late summer heat and dryness or are just done with their growing season, another species won't be. The mix gives you better control of your runoff, prevents mud/dust/erosion, overgrazing.
If you have very heavy clay-ey or less well draining land where alfalfa and clover will go bananas and choke everything else out, that's worth thinking about ahead.
Pasture with a lot of those legumes (alfalfa, clover) isn't always good for all horses especially small ponies and 'easy keepers' that get fat easily.
So there - with heavy clay-ey land you have to talk to your ag extension office and see what they can suggest.