I wouldn’t put more than 1 inch of sand depth in. Personally i think tanks look better with a real thing layer of substrate, so maybe 0.5 inches in depth?
As for plants, there are not many that can root in sand. Instead, do some research on Java Fern, Java Moss etc. These plants root against rocks and bogwood. And are normally a lot more hardy.
You should pop down and have a look when you next get some free time...
Aquarium sand is sold everywhere, and java fern and moss are easily available here in the UK.
it should still be easily obtainable. it doesnt go in sand, just attach it (or buy it pre attached) to bogwood or stones etc.
Ceratophyllum demersum does well in sand, it doesnt have so much roots but modified leaves to anchor it down in the sand. i have had some limited success keeping some vallisneria sp. in sand