There is a ton that I could write about that. And much I’d not even know to wright. Some mares need feed all the time to maintain a healthy weight. Some really don’t until late stages. Ration balancers are great because they give the needed nutrients without all the extra calories for those that tend to be chubby. I really like purina Omolene 300 mare and foal for my mares once they hit the 300 day mark or even a bit sooner. Alfalfa is great for making sure they get enough protein. Make sure they do NOT get any fescue, this will cause major foaling complications. Avoid the purina mini and pony feed. It used to be good but is now full of fillers and really not of any use.
Make sure you are very aware of all the different foaling positions and what to do for each position.
You don’t have to do any testing to confirm pregnancy but it sure is helpful. I’ve used all three methods, US, blood tests, and waiting it out. The only pee test that is accurate is the wee foal 120. There is a cheap knock off version that is all over the place. I refuse to use it because my good friend apparently had a pregnant stallion according to that test.
I’m not sure what you are planning on doing with a foal, but if you are planning on selling, you might make sure you have a good market. Also not sure if you meant plural stallions, but it can be pretty hectic to have multiple stallions especially with only one mare. Even one stallion can be a handful if someone is unsure about what they are doing. Also not sure if these horses you are looking at are registered but I would HIGHLY recommend testing for dwarfism and LWO. As long as one is negative you will not have a dwarf foal (which many confuse to be a really small foal when in reality it is a genetic disorder that causes major problems.) LWO is also known as frame overo. If both parents carry it and both pass it, you will have a lethal white foal and it is 100% fatal. It can hide on solid colored horses so it’s important to test/research.
There are a list of foaling vaccines. I tend to avoid vaccinating mine while in foal because a number of them have been known to cause abortions.
Make sure you have a good camera system. A lot of people do it without cameras and that’s fine, but cameras make a WORLD of difference. I’ve known people who watched in person and literally left just long enough to shower. They came out to a foal dead in the sac because no one had been there to break it when the mare didn’t get up fast enough.
Make sure you can handle your mare everywhere. That is something that cannot be stressed enough. It comes in great when testing milk or other signs. But if you have a dystocia a mare that is not okay being handled can become extremely dangerous. Even one that is used to being handled can become dangerous. Make sure you have a good vet around that can assist should you need it. It helps to also let them know when you have mare close so they may expect it or know what they are going in to.
That’s all I can think of at the moment but I will add more if I remember something else.