If you can find a ready made board that does what you need, they are almost always the best choice from a cost and size perspective. Most of the time I can't find the parts for twice the cost of such boards. As for learning how to build basic circuits like buck/boost converters, linear regulators and deciding when to use each, there are countless resources online, often as simple as typing your question into youTube's search bar. I'll recommend Jeremy Blum's introduction to Arduino for a basic overview of micro-controllers and simple electronic components found here: https://www.youtube.com/watch?...t=PLA567CE235D39FA84
For more advanced things I very hightly recommend Dave Jones EEVBlog and associated youTube channel. The content varies but there are numerious older videos going into detail on various things from a professional engineer from how components work to PCB design to dumpster diving and repairing electronics. https://www.youtube.com/user/EEVblog
There are many other sources, and any $50 electronics or Arduino started kit will get you through the basics. All that said, for the case of a couple amp ac to dc regulated power supply, it's very hard to beat just buying one unless you need hundreds of them that fit a specific physical design.
As a side note, you may not actually need that highly regulated of a power source to drive the motors. If your electronics use a typical H-bridge chip it may be possible to power the radio off of a well regulated supply which only needs to provide a fairly low current and power the H-bridge directly off of filtered DC before a regulator.
As a last note, if you're really interested in learning some electronics, I wouldn't even bother trying to adapt whichever product to work, but rather try starting from scratch, and build your own drivers and transceivers. Between Arduinos, ESP8266, and nRF24l01+ modules there is little that can not be done at really surprising prices.
