I live on a 40 foot cruiser stern narrowboat. When I was looking 2 years ago I got lots of advice here and people were great, as they are being for you. My input:
Not all narrowboats under £20000 are collanders! I paid £15000 for mine, and my surveyor was very impressed by its condition and said as long as I look after her she will give me many years of happy boating.
I sometimes find it difficult living in such a small space. Take into account the probably 10 feet of outside space, that leaves only 30 feet to live in. I used to live in a large extended semi house, and have always been a hoarder, so constantly have to shuffle things around or get rid when I get too much!
At only 40 feet long, my boat has everything I need, just! On entering at the stern, down the steps the settee is right there. Then there's the stove, then kitchen (double base unit either side and cooker and that's it). Next is the loo and shower, with small hanging space and drawers, then the double bed, then you climb on the bed to get out the front doors, and that's it! I would guess anything smaller than my boat would lose something, be it a permanent bed, the hanging space and drawers, or having to have the toilet sat in the shower tray. I don't think I could cope with anything smaller than my boat.
Regarding not wanting anything too big for your being a bit wonky, I've found that the smaller the boat, the more it moves around, and GRP cruisers are very wobbly compared to narrowboats. I've worked in a dry dock since buying my boat, so I've been in loads of different boats, and for someone who is slightly unsteady, I'd say bigger is better.
Good luck, hope to see you pass by someday! X
My water is heated by running the engine, my gas is purely for the cooker.