If you go to Petsmart (probably Petco too) they'll have a 10g starter kit. It's usually far cheaper to go that route than to buy everything individually. You'll get the tank, hood, light, heater, filter, and net out of it and all appropriately sized for the tank. Remove the carbon if you can from the filter media, some say it removes nutrients the plants need. You can just rinse the filter pad in tank water to clean it each week, and re-use it until it literally falls apart on you.
If you're going for eco complete because of the plants, you may want to consider skipping it unless you like the look of it. Yes it has nutrients ... but you can get the same effect using liquid and/or root tab fertilizers and they are far cheaper. Plus only substrate rooted plants will get any benefit out of them. Stem, non-substrate rooted (Java Ferns, Java Moss
, etc) and floating plants get nothing at all from it. A 10g isn't too bad for substrate though. You can't really mix sand, or put sand on top of the eco complete (or any gravel) as over time it will just settle down to the bottom of the tank.
The hood in those starter tanks likely have two incandescent lights. If you swap those to daylight CFLs from your hardware store (cheaper than pet store) you'll easily have 'moderate' light which will enable you to grow a wide variety of plants, due to a 10g being small go for 'midground' plants as your background and perhaps some floating plants (but keep them in check so the beta can go up for air).
I'm not sure who ate your snail ;)