is it wise to buy 1Kg dry FeCl3?
"Dry FeCl3" is an oxymoron. It's extremely hygroscopic. As soon as you open the package, it will be a sticky-wet, staining, semi-solid 1kg clump. Or more likely it will already be that way when you buy it. And despite it dissolves quite slowly, it creates so much heat that you must go even slower. So it takes forever and the FeCL powder will stick to everything. Trying to do this "neatly" is quite a pain. Liquid ferric chloride is easier to handle. If you have easy access to muriatic acid and powdered rust, mixing these together makes the same thing without dealing with "dry FeCL3." And the red rust will dissolve pretty much instantly in the acid.
Muriatic acid alert: In the US, Home Depot is carrying only this stuff by Kem Tek that masquerades as muriatic acid. If you read the ingredients, it lists 15% HCl. And 85% "not suitable for spray adjunctive." Whatever that means, it does not mean water. This stuff ruined my etchant.
Back to OP, ferric chloride keeps fine in a sealed bottle. But if you want to get the most out of it, you also want some muriatic acid. When it slows down, adding a little acid will make it work a bit longer. And when you get a lot of copper in it, aerating it will help. If you get a proper bubbly tank going, you can use it indefinitely; just add some acid and water when it clouds up.