A chest freezer seems like the easy purchase, but it needs a floor, an outlet, a route through the house, and a place where it can run all year. Fit the unit to the room where it will actually sit, then fit it to the food your mother buys.
Local option: New Plan Care gives families in the St. Louis area in-home help so a parent can stay home: companionship, help with bathing and dressing, meals, and overnight care. Call (314) 405-0887.
Measure the floor and the air
Measure the footprint of the freezer you are considering, then leave room at the back and sides for airflow and at the lid for it to open fully. Check the height with the lid up, because a low shelf above it can block the lid. Look up the manufacturer's rated ambient temperature range, which is usually printed on the unit or in the manual, and compare it with where the freezer will stand. A garage that gets very hot or very cold can strain a freezer that is not rated for that range, so an unheated garage or a sunny back porch may call for a different spot.
Check the outlet and the cord
Plug the freezer into its own outlet, not a power strip or an extension cord. Confirm the outlet is grounded and that the cord reaches without stretching. If the only outlet is across the room, plan the cord route before delivery day, not after.
Fit the door and the route
Measure the doorway, the hallway, and any turn between the curb and the final spot. A freezer that fits the room will not help if it cannot get through the garage door or down the basement stairs. Ask the seller whether the crew will carry it in or leave it at the door, and who moves it if it does not fit.
Fit the food she actually buys
Look at the shopping pattern before you choose a size. A household that buys in bulk and cooks in batches needs a different freezer than one that shops weekly and freezes a few bags of leftovers. Ask your mother, or whoever shops for her, how much they freeze in a typical month and whether the baskets will reach the items she uses most.
If you need a delivery, a haul-away, or an electrician to check the outlet, the Local help page is a place to start for the St. Louis area.