You need to be looking for something that will require a similar regime to yours. If your horse is a poor doer, and you get a good doer then you will have to separate them when the spring grass is in, or even for the whole summer.
If you have two good doers that is easier because they can both be kept in starvation paddocks together :)
Apart from that, for a companion choose with your head not your heart. Choose an easy to keep, easy to manage, easy to catch well mannered horse or pony. Unless you absolutely know that there will ALWAYS be someone around to exercise the companion, you might consider an oldie that does not need to be ridden, or a non-rideable for other reasons.
Get up, get going, seize the day. Enjoy the sunshine, the rain, cloudy days, snowstorms, and thunder. Getting on your horse is always worth the effort.