Well personally I think that light will be fine but it depends on how many and how big your plants will get. I think it uses older Samsung diodes (LM281B+ compared to the LM301B in my Spider Farmer SF1000).
Differences between HLG and other brands can be vast. They have features like extra infrared LED, UV LEDs, generally better construction, add on boards to add UV and Infrared to an already existing light… Bunch of stuff.
Other thing, when talking in terms of ‘how much can I grow’ we usually really look at the volume that we can fill with light. For example, my light is the Spider Farmer SF1000, 100 watts, will vegetate a 3’ x 3’ area, and flower a 2’ x 2’ area. You could put just one big plant under it, or multiple smaller plants.
Unfortunately I am not the light expert here so I really can’t tell you how much of an area you would be able to grow with that light…
As far as the other equipment dbrn32 mentioned, I would say the absolute most important thing would be the pH meter and pH up/down. You NEED, NEED, NEED to make sure you pH the water you feed the plant. pH is how alkaline a solution is. If the solution is too alkaline or acidic, it highly influences the nutrients the plant can take up. Incorrect pH can lead to crappy bud, and even a sick or dead plant. The recommended pH for soil is 6.5. I personally, and many of us here, don’t trust cheap pH meters, so most recommend the Apera PH20 tester. It is the best of the cheaper meters you can get. You also need to get pH UP/DOWN solutions to raise and lower the pH. Here is a handy chart to demonstrate the difference.
Also, I see that you said
Happy Frog isn’t a fertilizer, its a soil itself. I would recommend looking for a grow store near you and picking up some of it there as it is one of the best soils you can use, what I personally use, and is WAY over priced on line. Use that instead of the MiracleGro (I know you said you would let your Mother have it already)
Like others said too, the Seed starting tray is unnecessary, but will still work. Same thing with the heating mat, not needed either. A 15 gallon pot will make you a literal MONSTER of a plant so maybe want to return those and get something smaller… Here is what I do: Start germination in a cup of water for roughly 24hrs, by then usually a taproot has grown. Then plant in a Solo cup (16oz red party cup) cut some holes on the bottom too. Then after she is ready to transplant again, move up to a 3 gallon pot. Then up to a 7 gallon pot. There are a few reasons to do this.
1 .It is easier to water, since you are in a pot that is proportional to the size of the plant.
2. There is more surface area against the roots when you transplant, usually resulting in more vigorous growth.
Here is a handy little graphic to show that second point.
One last thing. Maybe don’t start off with that many plants to begin with anyways. Maybe start off with 2 or 3, so that way you aren’t overloading yourself with things to do, especially during feeding.
I know I missed something in there but this should be a good amount of info for you to absorb for now.
Edit: You are also going to need nutrients. I would recommend getting Jacks 321 formula. Let me know when you absorb the information above and we can go into the nutrients a little.