I was introduced to LAing this way. A LQS 60 miles away from me (hour and half drive through the mountains) had a similar deal. Take the class get certified and then you could rent time. I realized 3 things immediately. 1. I really liked LA quilting 2. I would never get any good at it renting time at a shop when I could only get there every few months. 3. The pressure of the hourly rental rate hanging over my head while I was loading/quilting etc would definitely have an effect on my quilting.
It was then that I started saving and shopping for my own LA. Before you even toss around the idea of water soluble thread, better make sure the LQS renting time on the machine is ok with it. It may just be a moot point if they won't allow it. You can practice your loops, C's L's etc but I can tell you right now it will be practice pretty much thrown out the window if you can't get back to practice more often than once every two to three months. LAQ is just as much about muscle memory as it is skill and talent. If you can't practice the designs enough to the point that the movements to make them become second nature to you it will be an effort in futility IMHO. May as well just use the LA for basting if that is what your ultimate goal is anyway seeing as you plan on doing your "real" quilting on your DSM at home.