[Squawkbert]

1) Welcome to APC

2) I think that, for the plants you list, 20W at 450nm will be adequate. If you get more ambitious (and want to grow Cabombas, Myriophyllum, Glosso etc.), you may want to upgrade later - lights, a CO2 source, regular ferts etc. etc.

2a) As you're starting out, you may want to consider adding a fast growing plant into the mix (Najas, Stargrass, Hygro, Wisteria...).

 

[Squawkbert]

That's always going to be a good plant to add to new tanks.

I've had Crypts, HM, moss, Najas and jungle val in my 5g tanks. They've all been pretty easy for me to grow (though jungle val gets very large and does not like Excel).