Your CO2 cylinder appears to be a 10 pound cylinder.
The regulator you have appears to be a single stage, single gauge regulator. You may want to upgrade to a dual gauge regulator so that you can tell how much CO2 gas you have remaining in your cylinder. Alternatively, you could upgrade to a dual stage regulator (this is different from dual gauge).
The solenoid appears to be fine for use.
You will also need a needle valve so that you can control your CO2 to a finer degree.
For more information regarding pressurized CO2 and parts, please take a look at the CO2 guide that I have written (it is linked in my signature).
As for your other questions, to start a planted aquarium, you do not need to drain the aquarium. If you are using gravel, it is fine as well; you do not need sand. Fertilizers (substrate fertilizers) can be added in even with water in the aquarium, if you wish.
CO2 will drop your pH slightly, but this is not of concern, as pH changes that are caused by CO2 will not harm livestock.
In terms of different pressurized CO2 systems, as you have realized, there are many different companies out there that provide similar products. You just need to do your research before making a purchase so that you know how to use it, etc.