1. Top and bottom: these will be plywood to help stabilize and waterproof the stand. Ill send you over a pic of how I have my plywood bottom setup after lunch.
2. Back: also regular plywood. I'll drill holes in it for the pipes and for electrical cords. Using 1/4'' plywood will work great here as its easier to cut your holes and cheap and easier to remove if later on you want to change the backing setup.
3. Sides: they come at this point. so they can cover the back on each end. I'm going to use bead board for this. I plan on making them removable with magnets. Do I need a router to make holes where the magnets will go so the sides will lay flat? Question why do you want to be able to remove the sides? Given the short width opening of the internal stand I doubt you'd be able to have much access to it anyway. I generally only have the sides removable if I need to get a sump ran though it, which in this case your location and width wouldn't allow anyways.
4. Front: this is where the nice stuff will go. Nice wood, trim, etc. I'm also planning to make the whole thing removable so the sumps easier to get to. I can cover the wood seams from the sides with the front. You won't need to have a removable plywood setup for this. What I plan on doing is setting up a removable center brace with the use a my kreg jig. This will allow me to have my front stationed while still allowing me to remove the "plywood" center brace with the use of one screw on top and bottom of the centerbrace board. All will remain invisible and look better than without once the cabinets are in place to prevent any cabinet gaps in-between.
My only problem here is how do I cover the side seams of the front? Another option would be to cut the sides a little longer so the front can fit between them and then use trim to cover the raw edges from the sides. What do you think is the best plan?
My front goes across the whole stand covering my sides. I don't use removable sheets so I can cover this gap with wood filler and paint over it. You wouldn't be able to see anything after its all done.