Yeah that filter plan sounds good to me. Plus, I know petsmart sells the 406 so you can get a discount. Maybe to play it safe, if you're going to continue to use the 404, you should buy a tune up kit. Like replace the rubber gasket to ensure no leaks.
fish do WAY better in a planted tank - with the exception of larger non-dwarf cichlids. Plants help by naturally filtering the water. They also provide an aesthetically pleasing environment for you to enjoy and also the fish. Fish live happier and with less stress in a planted tank. Actually natural sunlight is bad. It would cause algae in most cases. Instead, you light your tank with either T5's or high output LEDs. You can start a planted tank roughly three degrees of difficulty as follows:
Low light - commonly referred to as "low tech", easiest to do with low light tolerant plants. Light level is below 30 PAR (photosynthetic active radiation). Less ferts, no CO2
needed, rarely trim, and perfect for beginners. Most cases, no special substrate needed. Root tabs, maybe... depending on substrate and plants (i.e. swords or crypts). For examples, look up "Low Tech Planted Tanks."
Moderate light - slightly more difficult. Light level is 30 to 79 PAR. More options on plant species. CO2
or liquid carbon needed to avoid algae and keep plants healthy and thriving. Comprehensive fertilizer regimen needed. More difficult but can look lush. Examples fall somewhere in between low tech and high tech. Maybe look up medium light planted tank. Perfect for "nature aquascape."
High light - commonly referred to as "high tech planted tanks." 80+ PAR light. It's fairly difficult but not impossible. Just takes experience and/or a lot of research and reading to learn. CO2
injected and lots of ferts need dedicate balancing with light to avoid algae. But plants get real colorful with reds and many plant species to choose from. Fast growing so lots of trimming. For examples, look up "Dutch Aquascape" and "Iwagumi".
If you're interested in the planted tank option, start a thread in the planted tanks section here. Lots of knowledgeable folks here ready to help.