I would make an appointment at the vet to rule out a health problem. Also if your older cat does have the start of arthritis there is a glucosamine supplement you can get from the vet called Cosaquin that will help. My oldest cat takes that.

I personally wouldn't get the Littermaid self cleaning box, it's too small of an area for the cat to use, older cats don't accept change as easily as younger cats, and the noise when it cleans may scare her. Just keep your regular litterbox cleaned out daily.

If it were me I would just look at the local pet stores for a new regular litter box with lower sides, or even go so far as to get one of those dog litter boxes with very low sides, kinda depends on how much litter your cat throws around when using it.