I purchased my Tin Lizzie 18LS long arm four years ago and have never, ever regretted that decision. I made the payments but never considered taking in others quilts. Nor would I ever allow someone else to use my machine. It's too expensive and like someone stated before, it's your cost if someone breaks it. Besides...that MY baby :-D
I equally love piecing AND long arming but working full time doesn't give me nearly enough time for either but retirement is only a couple years away.
I started out with a mid arm machine but quickly became frustrated with the small "play" space. So i fortunately sold that for what I purchased it for and got my 18LS. There are a lot of great machines out there and you'll find so much support here and on the blogs for the different brands.
If you feel you need to do customer quilts perhaps you could limit it to doing pantos. they go pretty quickly and then you'd have more time to practice on your own.
Good luck on your decision.