Depending on where in the world you live will affect you depending on how cold it gets in the winter and how basic of a home you can stand. My first boat was 24 feet with a headroom of 5'5" and a portapottie. I never considered living on it but across the dock was a very happy guy living on his 25 foot boat. I did live on my 29 foot boat with only an electric heater and while mine could heat most of the boat it got cold in the winter. In the summer it was great because you can leave the hatches open most of the time, but in the winter I felt closed in. Fiberglass provides no insulation so if you water is cold so will your feet be unless you add carpet or wear shoes. Aluminum conducts well so a keel stepped mast is going to bring cold down with you. I have seen people wrap them to avoid the cold shock from touching it on the way to the head or v-berth. My mast is deck stepped so its not in the cabin.
Depending on your marina or wherever you dock, you have to consider the bathroom situation. The marina I lived in encouraged liveaboards to get and use portapotties and they had a place on the dock to empty the things and they had portable carts for people who used holding tanks. Sometimes the carts did not work. Fortunately my marina had great restroom facilities and I primarily used them whenever I could. Most showers use quarters or in Canada they use Loonies. Figure on average $1 per shower so thats $30 a month or less if you want people to stand further away or upwind of you. Some people join a health club or the "Y" since they usually have showers and if you are spending the money anyways.
Two boats to consider; Catalina 27 and Erikson 27. Both have a decent amount of space for one person who isn't claustrophobic to live and are decent sailors. I know people who live on both and you can find them between $5k and $15k. There are zillions of other boats depending on your budget, there is no upper limit on price.