http://www.discount-trailers.com/motor-carts.htm

I need a heavier duty motor cart and the model that support 315 lbs looks very nice.  I'm just wondering if anyone has done business with this company.  

Not sure how heavy of a motor you're talking about but a grocery cart makes a pretty good one.  Just add enough weight in the basket to keep it from flipping.

If you are talking a motor under about 75 or so pounds, a granny stand works wonders.  Here is my 50 pound Evinrude 4 deluxe.

Now, if you are talking 315 pounds, I would just get a hand truck, and mount a "transom" on it.  

I actually use to make these for a company, Estran Corporation.  We used to make the boat seat hinges of almost all models of lunds and a bunch of other companies.  Long story short, we made motor dollies and sold them to cabelas, bps, and other big stores, or other marine places that would put them in their own boxes.  I would advise against a motor dolly with the aluminum casters on it, Unless you plan on only moving it on a paved surface.  You cant push it around on the grass (so if you take your boat and store it behind your house on the grass, you cant push the dolly across the grass, the casters dig in the grass).  People used to complain all the time about it, so we made a model with 4 of the bigger "wagon" wheels, thing worked awsome, you could even do a couple mile portage in the boundry waters with it and never have a problem.  I have also heard of guys taking the wheels off of there old grill's (cooking grill) and rigging them on the dolly.  Estran is no longer in business, the large boating companies found it cheaper to have there stuff made "overseas" rather then in the states.  Hope this helps.