I think, while it may not be ideal that you are inexperienced, with the right help it is more than possible. Definitely learn from the previous owners and maybe contact a vet for more on their care as senior citizens. The owners should already have a vet and farrier they use so I'd get their contact info. They will know more about the horses and their needs as they've been caring for them.
Things to ask:
-date of vaccinations and type(rabies, nile, etc.) you also might ask the vet what diseases are most common in your area and do a little research so you know the symptoms to watch for.
-last time the teeth were floated(horses have to have their teeth filed so they don't get sharp and make it uncomfortable to eat or accept a bit)
-Last time the horses were dewormed (a quick search online should tell you the symptoms if you don't know what to look for)
-any prior health issues or injuries and the severity, effect on them as riding horses etc.
-What kind of feeding schedule they have been on, find out the type of feed any supplements, if you need to add water to the grain or if any of them have a history of choking or colic(choking:horses don't have a gag reflex so often times when they get something stuck in their craw it stays stuck and must be removed by a vet or someone otherwise experienced,colic: is a symptom of many things though it could be as simple as a rapid weather change, but is basically some form of indigestion and if left untreated can be fatal) you can google the symptoms of both and here are a few videos as well
More advanced cases and loud symptoms- https://www.youtube.com/watch?v=uB-o9e6_rxM
Subtle symptoms of colic https://www.youtube.com/watch?v=Q8Qiq5k3C9E
Symptoms of choking https://www.youtube.com/watch?v=qk47XCBvDNk
It might not be a bad idea to ask your vet what to do in case of an emergency until the vet gets there. Don't be discouraged though, these are all worst case scenario but it never hurts to be prepared. Also ask previous owners if the horses have ever coliced and what symptoms they have. Where I work i thought a horse was colicing because he kept throwing himself on the ground an thrashing but it was just because he wasn't with his pasture mate.Every horse is different.
-Ask the owners about providing hay during the winter, how much they get and feed etc.
-It might not be a bad idea to take a few lessons with a trainer or if the owners are willing to show you that's great too.
-Ask last time the horse's had their feet done, also get in touch with the farrier about anything you can do to help their feet if they are bad like using a supplement or a hoof sealant etc.
You'll need a set of brushes, and I find having basic first aid on hand is helpful. I keep an iodine wash, some bandages and vet wrap, thrush buster, a anti-fungal spray, and a topical antibiotic(great for keeping flies and bugs out of small cuts during the warmer months) in my kit.
Otherwise it'll just be a learning process with them, and know its never a bad thing to ask for help. You can ask most horse people and they'll tell you if for nothing else then the sake of the horse and of course you can ask here as well. Good luck!