One thing about stablizing the quilt before your freemotion it is that 9 times out of 10 you can then remove a significant amount of pins. This will lighten the quilt making it easier to move the quilt around as you free motion it. Diane Gaudynski reccommends stabilizing the quilt everytime before FMQ.
I do have both a walking foot and the sole for stitch in the ditch. Having the SITD foot is nice but with a little practice you can do nice ditch quilting with just a regular walking foot. One thing that really helps is having nicely pressed seams and nested seams. The last quilt I did I did not use the SITD foot. I used the regular walking foot. If money is an issue, then I would get the plain walking foot. You'll get a lot of use out of it. You can SITD or straight line quilt or echo quilt, or just plain piecing and sewing on your binding.
Just as an aside. I no longer pin my quilts. I use spray basting. It holds really nicely, I do the stablizing quilting and then my FMQ. The quilt stays together but is much lighter and easier to move around and I don't have to stop and start to remove pins.