As cart coordinator for CorgiAid I've seen the good wheelchairs and I've seen the bad ones. And believe me, there are bad ones out there. Expect to pay $300-$500 for a quality corgi wheelchair, new, 50-70% of that used. If it is $200 or $150, buyer beware.
A good wheelchair cart for any corgi needs to start as a supportive two-wheel cart. It should have a saddle, hard or soft, that the legs fit into. A support band in the abdominal region is NOT enough support. There are several carts out there in the "cheapy" category that do not have saddles.
The cart should be made to fit or be adjustable to fit your dog. Wheels should be not be trike wheels or wagon wheels. PVC parts are bad news- they break.
For a dog with DM you also need to consider use as he weakens in front. Look at the K9 Cart companies (one is in Maryland and one in Washington State; we have links at Corgiaid . Both have adjustable carts with good support and eventually front swivel wheels. Merlin used a K9 Carts west cart. Another good cart for DM is Doggon Wheels; the wheels can be slightly counterbalanced and then later front wheels added. Ruffrollin, by a former Doggon employee, has a similar design. And Eddie's Wheels has the best counterbalancing, either with a variable axle cart or adding counterbalance after it is needed (don't order it this way) to a regular cart. Their front wheels are not the best as they do not swivel but are okay for indoor use.
Please stay away from the cheap carts! A cart is worth nothing if your dog won't or can't use it; it is priceless if he loves it and gets two more years of mobility out of it. Isn't the difference worth ten dollars a month?
For more information about wheelchair carts and caring for corgis and other dogs with mobility issues, see "Corgis on Wheels"- the book- available at http://www.corgiaid.org/cart/corgisonwheels