Tim, for the substrate regular aquarium gravel at the smallest grain size is recommended by every plant authority I have so far read, and I have used this for 12 years with good plant growth. The small grain size allows the plants to root, encourages the necessary bacteria activity in the substrate, and is easy to maintain. Too coarse a grain and the plants sometimes have difficulty rooting (some plants like swords and crypts have very extensive root systems) and it traps too much larger detrius; too fine (like sand) and it can compact unless you are vigilant in preventing compaction. A natural and dark colour is best because it is more "natural' and the colours of the plants (and fish) look better. There are many fish that colour up better over a dark substrate.
I do not use enriched substrates, by which I mean a layer of dirt, laterite, planting medium, etc., under the gravel. Some do, and there is no problem doing so. But it is not necessary, and it does take a bit more work and care to prevent mixing the material with the gravel, stirring up dirt, etc., plus there is the aspect of additional nutrients leeching into the water column if not used by the plants. The latter effect is comparable to overdosing liquid fertilizer: if you have more nutrients in the water than what the plants need, algae will take control. That's why I do not favour planting mediums in the substrate; once it's there, you can't remove it except by tearing down the tank. In 1996 I did try laterite, but could detect no improvement in the plants compared to the other two tanks without laterite (same type of plants, same light, etc). No point in wasting money.
I do use root fertilizer, specifically Hagen/NutraFin's Plant-Gro sticks. There is also the Seachem Flourish tablets. I prefer the former because they last a year (compared to 3 months for Flourish), and are less expensive. And I have had incredible results with this particular stick. These, whichever, are only needed next to the larger swords and crypts. Both these genera of plants have extensive root systems as I mentioned above and they are heavy feeders. Non-substrate rooted plants like stem plants, Anubias, Java Fern will not benefit from root fertilizer because the roots are not in the substrate (Anubias, JF) or the root systems occur all along the stems (stem plants) and liquid fertilizer in the water is sufficient. All plants obviously pull the nutrients out of the water, mainly through roots but also through leaves; heavy feeders getting those nutrients close to where they grab them helps them utilize them.
Which brings me to liquid fertilizer. I use Seachem's Flourish Comprehensive Supplement for the Planted Aquarium and have for a year now. Previously I used Kent Freshwater Supplement with to me comparable results. The main thing is using a complete ("comprehensive") fertilizer. Plants require a number of different macro-nutrients and micro-nutrients, and in a specific balance to each other. Flourish Comprehensive has this balance (you can see the list on their website). Any product that does the same would be as good I'm sure. The thing to avoid is getting different nutrients (like iron, magnesium, potassium, copper...) and dosing ad hoc. This is not only very expensive, it is risky. There is the real danger than either something will be missing, or something will be in excess of what the plants need or can store, and that brings other trouble. I have myself once overdosed on magnesium and the plants developed holes in the leaves; I read on the Aquatic Plant forumn of potassium excess causing plants to stop taking up iron. It is safer to use a comprehensive fertilizer. Once a week may be sufficient, or twice, depending upon the plants, how many, light and CO2 levels.
A last point on CO2 (carbon dioxide); I don't add CO2 or liquid carbon, I leave the CO2 to the fish to provide, and I balance the fertilizer and light accordingly. In a relatively heavily planted aquarium you can have more fish than in the same one without plants. The thing is all in the balance. My approach is certainly more low-tech than high-tech; the advantages are less cost to set up and maintain, and because I am relying heavily on the biological actions of the inhabitants (fish, plants and bacteria) with little influence from me, it is more stable and less likely to develop problems. That's not to say other methods don't work, they do; but it comes down to what you want in terms of plants, fish, and layout of money to get it. My aquaria have been running like you see in the photos for 12 years with only weekly liquid fertilizer and yearly replacement of root sticks. And I can't begin to measure the beneficial effect this has on the fish.