I bought my first longarm in 2012, for much the same reason as you. I was tired of basting, and my current quilt (at the time) was killing my shoulders trying to stuff it through my domestice machine. I specifically told my friends that I was buying this for myself. I had no thoughts of "paying it off" as I have never in my life sent a quilt out to someone else, so there was no cost payback for me. It was the convenience, and I traded a kitchen reno for it (as in, I choose the longarm rather that having my kitchen renovated).
I am now - 7 years later - a full-time longarm quilter (quilt hi-tech). It took less than 3 months after I bought that first machine before the first friend said "I really, really, really need this quilt done and I can't do it. Pretty please?" So I did. And then another for her. And then a different friend gave my name to a friend of hers (who I did not know). I consider that my first customer quilt
I was working full-time at the time (in high tech) so I didn't push for a lot of business, but through word of mouth my business grew. It seems there is a lot of demand in my area - there are a LOT of longarm quilters, but then we also have 8 quilt shops with 1/2 hour of me, so it must a big business in this area.
Trying to remember your initial questions - if you want the longarm, (and can afford it), buy it with no expectations of a business paying it off. Then, if things happen you win, but you aren't stressing about it all.
Use your longarm until you get comfortable with it, before you even think about doing someone elses. The stress of quilting someone else's quilt, when things go wrong or you struggle with learning something new are not (in my opinion) worth it.
If you decide to start a business, you need to become a joiner. I belonged to 7 quilt guilds the first year I really started to push my business. And they got me a LOT of business. Show your own work every meeting (different quilt every time), and don't forget to say you have a business. Also try to get your customers to show their quilts and remember to say who quilted them. Sometimes a stooge in the audience to ask "Who quilted that?" helps.
Insurance was a pain in the neck. Apparently there are only 1-2 companies here (I'm in Canada) who will actually insure this type of arrangement. And I cannot get full coverage for the cost of my machines were something to happen to them. But as soon as your first customer walks through the door, you need to have insurance (I discovered that my original house insurance was completely voided because of the business, even if the claim had nothing to do with the business).
Basically, follow your heart. There is probably more business in your area than you think - especially if you have guilds and shops around. I have found fellow longarms to be more than generous about sharing information, ideas, and even clients (if they are overworked or not interested for some reason).
I hope that helps. Feel free to PM me if you have any more questions about my business in particular.