Buying aquarium plants online is typically just fine. It's, let's say, an adventure sometimes if you don't know much about the plants you are getting. You can read til you are mentally exhausted and still get mixed answers to your questions about light, space, and nutrient requirements. Experience pays, and if you don't have any with a particular plant you are interested, just ask here! Most of us in the planted tank club have tried many of the plants you are going to be interested in already and can give you advice. (It's not a real club, but you will figure out who is in it by responses. Byron is the Grand Poobah.)
Here are a few hints that will help you look in the right direction... If the description says one of these key words, you will know what to consider or not. I will say this: All of these are not always correct, but they are a "general" truth most of the time.
Slow Grower = Low Light Requirements
Low Light = Slower Growth
Rhizome = Plant that should not be buried in the substrate. It doesn't refer to a plant group, but rather what its leaves and/or stems grow from. Think ferns.
Fern = Slow Growing Water Column Feeder. Attach it to something, don't bury the rhizome.
Fast Grower = Requires Medium or High Light to grow best. It may still grow in low light, but often looks less appealing or just limps along.
Stem Plant = Medium/Fast Grower (see above)
Carpet Plant = High Light Requirements and likely CO2 supplementation
Sword = Heavy Root Feeder. It will do better with root tabs in the substrate near it, but in a well established aquarium may be fine without it. The bigger the sword, the more roots to feed.
Sizes and recommended tank positions (also generalizations):
Foreground = Small, low growing less than 3" in height. May be low and wide or short and compact.
Midground = Medium sized height up to 8"-10". Plant it in the middle ground for a graduated height increase from foreground to background. If it's leafy, it could get WIDE... leave room for width especially on these.
Background = Tall and/or wide. These will likely reach the surface of your tank in the 12"-24"+ category. Plant them in the back or sides unless you are looking to make a curtain that blocks your view into the tank
I wish someone had explained this to me, but I figured it out. Again, these are generalizations, so take them with a grain of salt (and check to see if they are compatible with brackish water before adding salt.) Have fun! Also, check with other members that are close to you as they may be willing to share with you or sell you some plants at a lower rate than online retailers. I personally trim and ship all the time rather than trash my trimmings.