For the longest time I struggled to keep chop fed and happy. I never had enough credits and would buy food/water as soon as I could afford the cheapest one. Most of the time I had to choose between giving my dog food or water when he needed both. If you look at the percentage return for food/water and the cost... Comparing all the items, the most expensive have the best and most efficient ratio.
I let chop starve for a while... Poor dog... Kept taking him on walks looking for coins and let my credits regenerate slowly until I could afford both the best food and water together. Once I fed those both to chop he hasn't gone hungry or thirsty. The credits will regenerate fast enough to sustain chop so long as you buy the best food/water and not spend credits on the frivolous stuff.
For the collars, to save credits, if you get gold on all three walks you'll unlock the gold collar for free. Upgraded without spending credits.
For the training, it is best to hold off on all training until chop is sustained by food/water and you've saved up enough credits to have a nice reserve