I would go with a canister filter Or even the oldfashioned hang on the back filter for a planted tank, an undergravel will get chocked with plant roots before very long, the airstones which run undergravels will put too much oxygen in the water and drive off any supplemental CO2 you attempt to add - especially with the DIY CO2 which is pretty weak in comparison to an airstone powered by a luft pump. I used to feed my CO2 output line right into the "intake" on my canister filter, when a bubble of CO2 would get released it would get sucked into the canister, blasted into tinier bubbles and come out of the canister output giving a fine spray of CO2 bubbles every 10 minutes or so. Make sure the output from the canister is beneath the surface of the water to avoid agitating the surface which drives off the CO2 and artificially infuses oxygen in the water which is the oposite of what you want to do in a fully planted aquarium.
Yes, definately get the plants happy first then add the fish. You will not need to introduce any chemicals to start up the bacterial cycle if you are using live plants. There will be plenty of live bacteria on the plants which will breed and spread over all surfaces in your aquarium and filter, just give it a little time, ading a tiny bit of flake food will help speed up the bacterial population but only use a pinch, too much and the algae will kick in, blocking light from the plants and they will die leaving you with a green soupy mess!
When starting your planted tank go for fast growing plants at first such as Hygrophilla, Cabomba, etc. avoid things like Java Moss and slow growing rosette plants like Amazon swords, etc. because when the tank is cycling algae will be doing it's best to destroy your work thesefast growing plants will suck up all that nutrients and can be cut and immediately replanted next to the stand you cut them from so keep the water pure while the litte eco-system is getting settled. After a month or two of this you can start to remove the excess plantings of the fast growers add the slower growing "show" plants like colorful water lilies, amazon swords, etc.. Planted tanks are so fun, just talking about it is bring back all the old memories! ...it's only the water changes I don't miss!
Fish and plants by mail is fine if the shipper knows what they're doing. You can get live plants and fish from
www.aquabid.com it's like ebay but for fish! I got many of my killifish off there back in my fishkeeping days since they are next to impossible to get at the fish shops. I like Killis because they've very small but most are as colorful as saltwater reef fish and much easier to keep! Most killis are Annual meaning they don't live long but as long as your aquarium doesn't dry up like their pools do in the wild they will live 2+ years. Aphyosemion australe is an awesome non-annual species and is about 3-4", very freindly, bright orange, non jumpy and will even eat from your fingers. They also spawn like mad, if you would like to try raiusing babies with hatchling brine shrimp these guys are great fun!