Get POA set up for both of them for finance and health
Key safe
Keep house in good repair so it can quickly be sold if necessary - including garden
I personally would get them used to having a cleaner and gardener and maybe making the cleaner a sort of housekeeper but also company.
Keep on top of them using tech to contact you if you can - my Dad was v good at this, MIL terrible and we had a nightmare keeping in touch as it was literally by us writing to her she couldn't get out to post, had hearing issues so wouldn't use the phone so all we could do was write and say we were visiting. Neighbours were helpful but she also had a thing against them. I would also get two or three locals you could contact to do checks or pop in for tea etc. But also you can call on if something happens.
Make sure you have local numbers for gp, hospitals, health/social workers etc
I would also research local homes they could use if necessary, talk to them about favourite items and furniture they may want with them
DO house clearance and start now for things they don't need - close down some of the uneeded rooms, cull books, clothes, bedding, kitchenalia etc.
Check all floor spaces are clear of trip hazards, that all bathrooms function properly up and down - are there downstairs show - walk in? Maybe think about how high furniture is or isn't so they can get in and out easily.
Sort security (and cameras) and also things like get electrics checked like you would for a rental, fire alarms, smoke alarms, carbon monoxide detectors etc