I agree with I Would about commercial carts. I built a couple of PVC carts for my wife and myself several years ago, and enjoyed the process of building and using something I had built. They do break down for storage in kayak hatches. With the commercial ones we use now setting up is just 1. unfold the frame, 2. put the wheels on the frame, 3. put retainer pins on the axle to hold the wheels, 4. strap the kayak on. Whenever we paddled with a group and a carry was necessary my wife and I were always the last ones ready for the carry due to the complexity of assembling our homemade carts.

If you still want to build one yourself I can send you a file of the seven page illustrated assembly instructions I made for my wife. They should give you enough information to build that design yourself, too. Just send me your email address. Mine is on my forum profile.

Your other question - wheels. I found some inexpensive pneumatic wheels and tires at northerntool.com.