Jay Newirth has developed a circuit board which will allow you to use a meanwell power supply to power a Model III. You can purchase the meanwell power supply, the circuit board, and/or the cabling a la carte, or you can purchase the entire set, all on https://www.plaidvest.com/newsoft/.
If you are willing to not have a TRS-80 keyboard and would accept something more modern, the folks over at Plaidvest Software have noted that you can install a Wireless USB Jelly Comb KS037 into a Model III (once you take out the existing keyboard) with a NEWKEY/80. They also have a Thingverse to manufacture a keyboard support if you glue a washer to the underside of the keyboard.
If you need a replacement badges for your Model III, NewSoft sells them for a very reasonable price.
Power Connector Assembly
If you would like to use a standard computer plug instead of the one permanently fastened to your Model III, Newsoft sells a repalcement power connectory assembly. Note: Will require you to open your Model III and do some disassembly and re-wiring.
If you would like to repaint your Model III, you might want to read the How to Paint your Model III or 4 guide by Patrick Bureau, downloadable Here.
For hard drive and floppy drive replacements, visit: