Hi Mark - it's looking good! Just two comments. The wheels might be a little small. Some of the paths are very uneven and if you walk in mud, a larger wheel will be easier to pull/push.
Also, with small wheels it is quite low and the stopper in the front could create a drag on rocky or rutted paths. Is there some other way to design a brake/stopper in the front?
This guy made a push/pull cart and you will see that the wheels are fairly large. He said that the axle broke a couple of times which neccessiated repairs along the way.