First, if I haven't already welcomed you in another thread...Welcome to Tropical Fish Keeping forum.
Now, to your questions. A 5g tank is quite small, so this has an impact on the type of plants. You also mention low maintenance ("easy to grow and don't require a lot of special care" was how you put it). Plus having it in a window is going to make a difference.
. Plants will basically grow (and grow well) in any
substrate, be it gravel, sand, dirt or one of the special enriched substrates for planted tanks. The choice depends more on your preferences, the fish (borrowers like loaches prefer sand and will make a real mess with dirt), and how much work you want to have to maintain it because different substrate types can cause different issues, some not so good if you are not prepared for them.
Taking you at your word that this is your first planted tank and you want it to work without fuss, I suggest small grain gravel in a dark colour. I certainly would not recommend soil as your first attempt; it can be problematic if you don't know exactly what you're doing. Enriched substrates are fine but much more expensive, and as they are unnecessary to good plant growth the added expense in my mind does not justify them.
As for how much, this depends entirely on the plants. Substrate-rooted plants with extensive root systems need space; swords, crypts, vallisneria, sagittaria, aponogeton...some of these are too big for a 5g but crypts are some of the absolutely best for the setup you plan. And the smaller sagittaria and sword (Pygmy chain sword
) would be fine. So I would go with 2 inches of gravel, sloped or terraced (with rock or wood, rock works better) to provide more depth, maybe 3 inches, at the back and less in front.
The other advantage of substrate-rooted plants is minimal care; once planted they stay where you put them. Stem plants need regular pruning and the look is always changing as you do. Crypts, pygmy sword, Dwarf Sagittaria
all spread by runners or root shoots, and you can either leave the daughter plants or just snip them off. Floating plants are to me essential in such tanks to provide security for betta and similar fish, as well as dangling roots they prefer to browse through.
Fish selection is tricky in a 5g with a betta; a shoal of five pygmy corys would work, or shrimp. The Malaysian Trumpet snail is perfect, it will burrow through the substrate and help keep it loose which is good for the plant roots and for the tank health.
You will have fairly thick plants, and that is what you want with fish like a betta that are sedate and cruise among vegetation; they are not swimmers, and even if there was room for a shoal of swimming fish, the betta would not welcome them in so small a space.
One problem with window tanks is that plants naturally grow to the light, which is in back of the aquascape. Crypts are low-light plants though, and across the front this will not have much of an impact, and you want your taller plants at the back anyway. Shades on the window will be needed, or a "shield" for the tank for bright and/or summer days to limit the direct sun (algae issues plus overheating).
Potted plants are generally more expensive; the same plants without a pot are usually cheaper and will grow just as well. I have had both, and tend to buy whichever to get the plant that I want.
Hope this starts you off, do ask further questions if they arise.