Electric wheelchairs are great! Scooters also an option, but I found it harder to get into shops etc with a scooter, plus it's harder to use the bus and train (there's certain requirements around mobility scooters specifically).
What to look for and avoid really depends on her needs. For example, is she tall or shorter? Likely to need a seat width extender or not? What sort of budget? (mine was about £3k all in, a friend got one for just under £1k but it's much more poorly made and less robust).
Generally, consider battery life, the type of terrain it needs to be able to cope with, and cushions/ back support. Something I see a lot in older people starting to use wheelchairs is a lack of understanding of proper pressure care. Even if she's not using it full time, it's important to have an adequate pressure relieving cushion and lumbar support as required, which might involve purchasing separate cushions and swapping out the standard ones which tend to just be a foam block on the part-time usage ones.
I'm assuming as not for full time use you'd be looking at the folding, lighter weight variety. Would you need to get it in the car at any point? If so, think about how much you can reasonably lift (for context, mine is on the heavier side at 27kg and can be lifted alone by someone who's tall, young, strong, and fairly fit, but needs two people or a ramp to load in most cases). There's also scooter/ folding wheelchair hoists that I know give a lot of independence to those wanting to take them further afield independently.
My recommendation would be to go to a mobility shop together and ask to try out different options. Some companies will also do home visits, where they bring a van with a few pre-selected options to try out.
In terms of keeping dry, the wheelchair mac things are a bit rubbish if you're able to get in and out of a coat with sleeves. I have a dryrobe adapt which keeps me very warm and dry, but it's pricey. Any cropped or full length coat should be fine, especially if it's one she already has (no fun having to change styles and start using a mobility aid at the same time). Keeping legs warm is important, some people use blankets and/or something like the Bundlebean leg cover which is waterproof and fleece lined.