I'm just getting into planted tanks myself, but I'll try to help you out.
If you are going with a planted tank (and more than just one or two plants), you will need to supply nutrients. I agree with planting before the fish are in, there is nothing wrong with that idea. Plants are good because they fight algae growth, and can look very pretty in the tank.
If you are planting anything above sparse, you will want to look into providing nutrients. From what I know Iron, trace elements, CO2, and light is pretty much all you need (but also, my LFS told me not to worry about Iron?). A few plants can sustain themselves from fish waste, if you have enough fish, but more than a few and you need to look at your conditions.
High light? More nutrients are needed (i.e. above 2wpg). Your fish will not provide enough CO2 for your plants, or other nutrients, so you need to add co2/nutrients to your tank... your plants might grow if you don't add anything, but algae will grow even more. If you have too many plants, then they will all die from lack of nutrients over time.
Picking your fish to your plants, vice versa. Well, unless you want to have $40 worth of shredded plants, research what fish you might want and what plants. Do your best to pick plants that will not get shredded by the fish you want. I found that my chiclids (mix) love to shred any plant that does not have a 'stiff' stem (other than java moss). My african swords work very well, so do the onions, and some other type of plant that I have (forget the name, but it is stiff stemmed with small leaves). Also, I have a pleco, two catfish, and black skirt tetras.
