I mean, 18vac is like, really simple.
If you wanna go super high end DIY, just get one of these, throw it in an enclosure with a IEC on one side and whatever plug in the other.
BOOM. Easy.
AC to AC is EASY. All you need is a step down transformer.
AC to DC....less so. Old school is to use a step down transformer into a rectifier bridge, but that results in an output voltage that can float around based on the current draw of the circuit. Regulated linear power supplies are pretty easy to design, and can be extremely quiet. BUT...they create a lot of heat and are VERY inefficient.
Then there's Switched Mode power supplies...and you basically need an EE degree to design a good one. Properly designed, they can perform with very little noise and can be very efficient.
Personally, I think the best way to go about differing voltage requirements is to utilize different connectors for each input. Or...if that isn't an option, use some good 3x heat shrink and color code the ends. Hell, one could even use resistor color codes if one was so inclined...
Or there's always label makers that print on heat shrink tubing. My dymo rhino does that pretty well.