Complete kits for me with at least a parts layout. At the very least the PCBs.
These are all fairly simple, classics and in the public domain (AFAIK):
Overdrive - Tube Screamer
Phaser - MXR Phase 90 (or Phase 45)
Compressor - Dan Armstrong Orange Squeezer
Envelope Filter - DOD Envelope Filter 440
Distortion - ProCo Rat
Fuzz - Fuzz Face or Big Muff
For overdrive, you might want to look at either of the two DOD 250 overdrives, which with a few parts swaps can also be a MXR Distortion Plus or a Ross Distortion.
The Tube screamer can be socketed, allowing for a variety of different chips to be used. There are so many common mods for the TS, that it makes it quite a good one for a workshop.
You could also look at some simple switcher pedals - AB, ABY and a loop switch.
I like the idea of dirty blues tone, but I have no idea which pedal would be best suited for that.
Most commonly a Tube Screamer.