Are you currently working i.e. would you have to give up your current job? If you would just paint in your spare time, it definitely makes sense to try and make a career out of a hobby and talent.
I'm still learning to stop myself when I'm getting manic, as I also come up with the most outrageous ideas. I recently bought a pet chameleon during my energetic spells - an idea I'm luckily not yet regretting, yet it was really rash.

I definitely second the business plan idea, as it helps you to structure your ideas. Remember that a business plan is always a WIP - and if it stills looks attractive after a few reworks, you must be onto a good thing!