I agree with Peckish that the certification class is an ideal way to expose yourself to LA quilting. My first exposure to a LA was doing the certification to rent class at a shop over an hour away. It was a certification class on a handiquilter. It definitely gives you feel for if you will even like longarming. Usually they cover loading backing, top and batting on the frame then go over the basics of the machine (on, off, needle up down position, stop, go, etc). Then they usually have you try several techniques. In my class it included following a pattern design with a laser light (much like doing a panto, only it was just a small block pattern and we worked from the front of the machine) following a marked line (like a stencil), doing a little FMQ and we also got to try "groovy boards".
I would call each place that offers and ask the following questions:
Is the class all hands on
How many students per machine (in my class there were only two of us so we each shared the one machine head and got to practice each technique) Ideally the ratio is one person per machine but two is doable.
What things are covered in the class.
It really doesn't matter what machine you try this out on as you can try different machines at shows and if you seek out dealers in your area, many allow you to test drive the machines in their showroom without all the distractions and crowds of a major show. As I stated above my first class was on an HQ but I ended up buying an innova.