Long story short I'd say go for it, if you have the time and money, you clearly love the ponies. I do have a lot of questions though that might just be annoying or might help? Feel free to ignore if the former!
I assume by 'taking on full time' you mean full loaning rather than offering to buy them from her? It might be worth offering to buy them, or at least getting an agreement you'd get first refusal on any sale, if you are going to put a lot of work into them, just as even with older ponies, if/when owner realises how much they're worth there is a risk of them being sold out from under your feet and you getting nothing, I know no-one thinks their lovely amicable arrangement will ever turn sour with something like that happening to them but it does.
Do you have a loan agreement already, if not you must do one, if yes then make sure to amend it to reflect new arrangements.
Are the ponies ridden or non ridden? Will you definitely have time to ride/work with both regularly, would you want someone to hack out with for instance, perhaps you'd like to bring along a friend to ride, because if so you should agree boundaries around that with the owner especially with them being kept at her home. Ditto around taking them off site and bringing in instructors if that's something you'd want to do.
Definitely cover what the owner's intentions would be in the event of her death or also if she has to go into care. Also the horses themselves aren't spring chickens (sorry!), what will happen if one dies, will the owner be prepared to get a companion for the remaining one (she probably doesn't want to get into an endless horse cycle) or for you to move the remaining one to somewhere where they can have companionship?
Also for your own peace of mind you probably want to think through what you'd do in the event of a change of circumstances meaning you could no longer loan them - I can see that being a really tricky situation for the owner if you've been the only one with responsibility for some time, and the issue with them being at owners home as opposed to on a livery yard is it might be difficult finding someone nee to take them on especially a few years down the line - worth thinking through that contingency if so. I assume you already have holiday or sickness cover in place? Or did you and the other part loaner sort this between you? If so worth getting that sorted now.b