We have seven thoroughbreds and all of them live out 24/7 365 days a year. They range from a 6 month old filly to an 18 year old mare. All of them do just fine. However, in the winter and summer they tend to need more food because in the winter they burn of huge amounts of calories staying warm and in the summer they burn off even more than that sweating/keeping cool. They all seem to enjoy it though since they can graze at whim and run around if they feel like it. I find that it helps keep ulcers away and keep them healthier over all and wouldnt change it unless I absolutely had to.
We also don't blanket ours (except for the 18 year old) in the winter, but we live in Texas where it rarely gets below 20 degrees or so even at night.
My TB also lives out all year (North-East Scotland too, where we get two/three feet of snow for two months a year and temperatures down to -20. He needs LOTS of food, as he's a poor doer, so he gets lots of hard feed with plenty oats and sugar beet in the mix, and ad lib haylage through the winter. He also needs plenty rugging, but then again I do clip him out for work. In the worst of the weather, he has a 300g stable quilt on under his 600g winter rug and 150g neck just to keep him cozy!
If you're just keeping this horse without any intention of riding/schooling/showing her, than you can, like my previous Vet (too far away now) kept his herd of ~40 SB Broodmares in a stand of thick trees as their shelter.
However, you will have problems with wet, soft feet if she doesn't have a decent shelter to dry them. I know this bc I kept horses outside WITH a shelter for 14 years, and I used them for lessons 10/14 of those years. I am much happier now, with stalls for icy weather in the winter even though my 3 spend more than 6 months just turned out with their 16 x 19 shelter off of the barn. One slip on the ice and you'll be paying for an equine chiropractor, just so you know.