If you are starting from nothing: grinder, tray, and one thing to smoke out of. Those three cover it. Everything else on this page is an upgrade to one of them.
If you are upgrading one piece: make it the grinder. A better grinder improves every session. A better ashtray improves none of them.
On trays: buy for the lip and the width. A flat board with no edge does not catch anything, and a tray too small to work on is a tray you stop using.
Glass or silicone: decide by where it lives. Glass for the shelf at home, silicone for anything that goes in a bag. That is the entire decision.
If discretion is the concern: the storage is what solves it, not the pipe. A lined bag does more for smell than any accessory choice you make upstream of it.
If it is a gift: build a small set rather than buying one nice thing. A grinder, a container and one novelty piece covers function, storage and the joke. Three variations on the same object does not.