If you go with an energy star rated door you will get the best R values for your door. It is more than the glass, it is the construction of the frame and door body as well. Some of the more expensive models (Anderson and Pella) use a vinyl clad construction in which wood is wrapped by vinyl. Results in a more solid door. But if it qualifies for the energy star rating you should be OK. Low e and Argon I believe are more effective in hot sunny climates.Residential Windows, Doors and Skylights : ENERGY STAR

Thanks for the welcome! See picture (sorry for the poor quality, only one I have at the moment); the existing sidelight is 100% glass, so no type of frame or surround exists outside of the thin trim holding the glass in. My thought was to buy a sidelight glass only replacement kit, then fabricate a frame (that would be the size of that full glass pane currently installed) for that new piece of glass to mount to, then finally mount it in the opening of the current sidelight. I don't know if a project like this has been attempted, I haven't found much about it online. Sorry if I'm being unclear.

You'd only use hardwood on the inside if that's what you need to match the rest of the interior woodwork, otherwise pine or poplar would be fine. Basically you'd rip out what you have and build framework to hold your new side light. I'm just a painter, the carpenters should be along later