Ok Crickets are perfectly fine for a pyxie to eat. I don't think it's proven anywhere that pyxies digest roaches better than crickets.

I would not skip out on a thermometer or hygrometer. These things are very helpful no matter what herp you keep. Adults should be housed in a container with a floor space 6-8 times the size of the frog. These frogs spend a lot more time in the water than pacmans and are far superior swimmers. Many people recommend a half and half setup for their pyxies(half water half land) this can be accomplished in a few ways. Most people use a large water dish. A pyxie water dish should be deep enough for the frog to fully submerge but at the same time shallow enough for the frog to be able to stand.
I would recommend a submersible water heater for the dish.
$30 is not a bad price for a pyxie. Just makes sure it's a true giant adspersus.
Many newbies end up with a dwarf which was sold to them as a "giant".