Building my sub-domain and folder structure ??

In the past, Google treated subdomains as separate websites at least according to the SEO thinking at that time. Today, I'm not exactly sure. Some people say that they are treated as one site, some say they are treated as different sites.

With that said, I'm really not sure why you would put the content you are describing on a separate subdomain. Obviously you know the site best, but I personally wouldn't use subdomains for separating products like that.

If it were me, I'd use a folder structure such as:

example.com/farming-equipment
example.com/double-glazing
example.com/office-cleaning

So right there, when you're getting links to your home page you can link from there to these major categories and pass link juice to them directly.

Then for articles, I would like like:

example.com/farming-equipment/tractor-maintenance-monthly-checklist

You would link to that page from example.com/farming-equipment, passing link juice to your second tier of pages.

For your sales page, I'd again use the same structure:

example.com/farming-equipment/hoes.htm

I think it's best if you remove any unnecessary words from a URL structure. So using example.com/farming-equipment/catalog/hoes.htm might make logical sense, but the user doesn't care since they'll be clicking a link and I think from an SEO perspective it's stronger to remove catalog.

So on example.com/farming-equipment you have links to all of your sales pages, and links in a sidebar or some other location to your farming equipment articles. If you had a ton of articles you might consider linking to one page and then link to the articles from there.

Again, you know what you're trying to accomplish better than anybody else but based on what you've said in the post the above is most likely the approach I would take.