I started with plain muslin, then charity quilts. Then my own. The customer quilts came after that. Many of the quilt shops have longarms available to rent. It's a great way to see if it's for you!
Just as important as being able to quilt, is the ability to run a business. Are you a self-starter? Self-motivated? Can you manage the books, pay the taxes, local/state/federal? Can you manage your money, quilting is sometimes feast or famine? How are your customer service skills? Do you have room for a machine that takes between 12 to 14 feet in length, plus room to go around at both ends (nobody wants to do stop-drop-and roll!!!)? Do you have between $10,000 (if you find a used machine or someone to go in with) and $20,000 dollars plus the costs of thread, batting, and yes-even we quilters have our specialty tools, and you can't buy them at Jo-Ann's with a 40%-off coupon!!! I took a ton of classes at a quilting show. Pantos are between $15.00 (if you're lucky!) and $30.00, and you will want WAY more than 1!
There are shows where the quilting of the quilt is the star, as apposed to the pattern and piecing (they play an important part, just not #1). It's not cheap, but there are classes and are a great way to learn techniques. Figure $30 to $50 per class, plus registration, hotels, meals, and transportation.
I personally have only done a stipple once (Happy to be Scrappy). I haven't done one since. I LOVE ruler work (also in Happy to be Scrappy), but I broke my wrist in a nasty fall on Valentine's Day. I'm still trying to get my stamina back for hours of ruler work. I do lots of pantos and have pattern boards for clam shells and baptist fans. I also do freehand.
I don't mean to paint this negatively, I'm not. But there are many things to consider beyond "I want a machine." I'm sure my husband wishes I knew all these things going in. Me? I'm glad I didn't!!!! :P