Hi AngelBeann and welcome to the forum.
I'm glad to hear you want to give your betta a better home.
First off, I totally agree with Murka. Preset heaters are only good for extremely small tanks or areas where the temperature stays very stable (like the equator). Good choices are Fluval, Eheim Jaeger, and Marineland. I also have a Cascade heater that is working well.
Most 10g tanks come in kits if you get them at places like Petsmart and Petco. This has its pros and cons. Yes, the filter comes with instructions but usually the filter is kinda lousy. You're better off choosing your own filter. AquaClear is a good brand because it has an adjustable flow rate and is pretty quiet; it's the one I have. I personally didn't have a good experience with the Tetra Whisper filters but that's just me. Fluval is a good brand as well but a little pricier. If you can, get replacement filter parts while you're there. Most filters have three - sponge, carbon, and a biological media. Products like API Stress Zyme can help speed up the cycling process as I assume you plan to cycle the filter with Conrad in the tank.
I haven't heard anything about Eco-Complete. I'm not sure but it does look like the gravel in it is a little rougher around the edges so it could be possible for your betta to catch his fins on it. Small rounded gravel is really best as it's possible for food and waste and even your betta's head to get caught in in between larger rocks. Rinse new substrate well before putting it in the tank 'cause otherwise your water will be cloudy.
Probably the better bet is to make sure your plants have the correct amount of light. Most of the common plants grow well with moderate to bright light. As Murka said, Java Fern, Java Moss, Anubias are all good plants. Hair grass is also good and Amazon Swords, too. Even if you don't get a tank kit, your 10g should come with a full hood (usually black) with a light. That will help with your plant growth.