Ply is never the preferred substrate for any type of veneer covering, metal included. MDF and contact cement is the preferred method. Be very very careful with handling the stainless. It's not at all easy to get the dents out. It's a very unforgiving material. I wouldn't use a ceramic tile front edge on it anywhere either, or use a topmount sink. That 100% defeats the primary reason to use metal for a counter: seamlessness. You might as well be doing laminate as do that. You're not getting any of the benefit of a metal counter at all, including the integral sink. You want to go down into the industrial section of town and find a metal fabricator who can create a folded edge for the counter. They'll use a break to bend it over and you'll just glue it to the substrate. I'd get them to also do a 2" backsplash rather than doing the tile as well. They can weld the sink in, and you'll have the total seamlessness which is why people do stainless counters.

lilafter:

If you're insistent on doing stainless steel DIY I would suggest using Chemetal. It's like plastic laminate only metal. You'll get the look of metal without needing any of the specialty trades. Depending on your layout and the size of the Chemetal sheet(s) available, you may or may not have seams. Don't forget a Karran seamed undermount bowl.

Here is a link that might be useful: Chemetal