I don't know too much about e-bikes, but I have a feeling that you're probably going to be disappointed with the performance of those considering the prices on them. When I was looking into them a few months back just out of curiosity, it seemed to me that even the low-end electric kits (ie: no bicycle, just the wheel/battery) sold for over $1000, so a $500 price tag on an entire bike+motor seems fishy.
That being said, the weight limits are probably due to the wheels. Bicycle frames can withstand an incredible amount of weight, and the weakest part is always the wheel. For a Clyde, I always recommend having custom wheels built. It may behoove you to buy a kit (motorized hub+battery) and have a bike shop build that into a custom wheel for you. I don't know if there are any kits out there that offer higher spoke counts, but you'd be best looking at 36h hubs/rims, and avoid anything with 32 holes or less.
Another thing to consider is that since we are clydes, we will get less benefit out of e-bike motors. Laws of physics state that Acceleration = force / mass. The more mass there is, the lower acceleration there is. Additionally, hills will be an even larger concern. The formula for potential energy (ie: the amount of energy it takes to lift an object straight up from the ground) is:
Potential Energy = Mass * Gravity * Height
The more mass you have, the more energy it takes to go up the same amount of height as a lighter person. This will have negative consequences on battery life, along with potentially stalling the motor if you attempt to go up too steep of a hill without pedal assistance. So simply put, we clydes, if we want to seriously consider e-bikes, need bigger batteries and stronger motors... which means we'll have to pay more to get usable motors.
Just some things to take into consideration.
Side Note: I eventually abandoned my quest to tinker around with e-bikes, because I felt that it would offer too great of a temptation to let the bike do all of the work. I'm glad I did, because now I can go long distances on my bike, and never feel the need for electric assistance anymore.