I totally agree with lump or high-quality extruded charcoal. No grocery store or big box store briquettes. No briquettes, period!
I personally prefer torch or newspaper/chimney for lighting over even the best of the squares. No lighter fluid, period! Especially in something that seals up as well as a kamado, you'll never get the chemical scent out.
Get a good butcher. A real butcher, if possible. No matter how good your technique, you'll never get outstanding results from warehouse or grocery store meat. Good and very tasty, yes, but outstanding, nope, not even their "Prime"
While a chef that grills a dozen or more steaks a night can easily know the difference between medium rare and medium just by how the meat feels, most of us can only get sort of close "by feel" Cook to temperature and you'll be much more consistent. Who cares if it's called "medium rare" or "rare" -- I know from my sous vide experience that my wife and I like our meat at 52° C. Remember that there will be carry-over cooking fro the residual heat, perhaps up to 5-10° F.
Grilling burgers to a specific doneness is difficult even for professional chefs. Best I can do is three levels, not done yet, done, and "why would anyone want meat cooked more than medium?" (with exceptions certainly given for reasons of religion)
Always let your meat rest for 5 minutes or so after it comes off the grill, before cutting, so the juices and heat come to more of an equilibrium
Searing doesn't "seal in the juices" as many claim, it's long been debunked as false. It does make the outside of the meat taste magical, but you can do it at the start or the end of the cook. As it is harder to get a kamado to cool down than to heat up, I personally sear at the end of the cook.
Agree, can't beat the ThermoWorks Thermapen. I've had them for years and wouldn't use anything else. If that's beyond your remaining budget, the CDN Pro quick-read thermometer at around $15 (Last one I ordered for a friend was a DTQ450X, they change the model numbers a bit from time to time)
You might want to consider a year's membership to America's Test Kitchen, watching some old Good Eats episodes, or both. The basic cooking techniques for meat, poultry, and fish are dependent on heat and time, wherever the heat comes from.
Edit: On the baked potato, easiest in the oven or toaster oven rather than the grill, with or without the time-saving shortcut of a brief bit in the microwave first.