A walking foot won't hurt your machine at all as long as it is the proper shank size and has the correct width needle opening for your machine.
Your Janome is a low shank machine with a slightly wider zz width. Be certain to get a walking foot specifically for the horizontal/drop-in bobbin Janome machines, or it won't line up properly with your needle plate opening.
A walking foot simply reduces the resistance between the bottom of the presser foot and the top layer of fabric. This keeps the top layer from "growing" and helps prevent shifting when sewing multiple layers.
Now- as far as industrial machines, they are intended for high volume production work as one might find in a factory. Some are made to just sew shirt weight fabric, not heavy stuff, but they can do it well at a lightening fast speed for multiple 8 hour shifts every day, week after week. They are most commonly more task specific than our home machines which are "jacks-of-all-trades" in comparison. So, if you want an industrial machine for sewing heavy materials, be certain to get one made specifically for that. An example would be a Tacsew T111-155
http://www.allbrands.com/products/abp03801.html?ppc=cjpdt Not just any old "industrial" will do.
Leather- any decent home machine can sew CLOTHING weight leather (think of a skirt or pants) with the proper needle, presser foot and sewing techniques.
As far as sewing canvas, it would depend on the number of layers and the weight of the fabric. My daughter hems jeans regularly with her Janome 4052LX and as long as she presses the thick seam well, uses the proper needle and keeps the presser foot level approaching and over the "hump", she has no problems. Now, if she was doing this for hours every day, it would take it's toll on the machine in no time. Once every few weeks- no problem.