If you search just on the manufacturer's name how many results are there? If your site jumped so quickly in the results without links I wouldn't think it's too competitive a name. Now just because G shows 0 links it doesn't mean they don't see any links pointing to your site. They probably see just as many as Yahoo does. They just don't let you know.
If it's the links that are resulting in the rank then those links are still going to be in place if you change the design. I would think as long as you keep the same page title and have some text on the home page you'll still rank well and pull traffic. Hard to know without the specific. Feel free to PM me a link if you want me to take a look.
You yourself mentioned you don't like the site now. That to me says you know it needs to change, but what's stopping you is where the home page ranks in Google. That rank isn't your end goal. It's a means to an end. If you think the site needs improvement then improve it. If you don't want to lose the ranking save the current file for the home page. Then make any changes you think you need and see what happens. If you lose all your traffic and aren't making any more sales, put the old home page back. Your ranking would come right back if they're the result of what's currently on the home page.
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