I have had several FW planted tanks over the years and still have one (29 g). The things that I've learned over time:
CO2 is necessary. I went for a long time with the DIY yeast generators. Major hassle and mess. Bought 10# CO2 cylinder, regulator and bubble counter two years ago and life got a whole lot easier. For a 125 I wouldn't even thing of not doing it. For any big tank with lots of plants, I'd recommend a pH controller to control the solinoid...I just use the same timer as the lights.
When you get the CO2 injection set up properly you can actually see O2 bubbles created on the leaves of the plants.
I've used several types of CO2 reactors and the best that I've found is the Aqua Medic reactor. It is a spiral within a 6" long tube...you flow water slowly through it and the CO2 bubbles take the long spiral trip in contact with the water. Works like a charm...nicely made, easily opened for cleaning. Also not much more expensive than some of the cheapies I'd tried before.
Plant it heavily to start. Just the opposite of a reef. If you don't have a bunch of plants right away you'll fight algae forever. Start with easy plants and then supplement or replace with more delicate ones.
Buying one or two plants from the LFS is okay but I've found that nobody stocks anything but the most basic. For the large initial planting or for exotics, you need to order from a specialist. Just like corals, shipping is outrageous for one plant, but not bad spread over a few dozen. Two that I've had good luck with are trueaquariumplants.com and aquabotanic.com. Excellent quality and selection at both. I'm sure there are others now.
Lighting...I've never tried halides so I can't help there. I have always avoided the plants that have very high light needs. I've used 6700K PCs with good luck. Tried 10K and actinic 50-50s without luck--helped the green algae more than the plants--I just switched back and am trying to recover.
Flourite makes a great substrate but it is expensive. You'll need to start supplementing with fertilizer anyway after about 6 months so I'd just use simple gravel and supplement from the start. I've used Flourish as a liquid fertilizer for years. Others probably work well, too. Getting fertilizer "pills" to place near the root systems of swords and other rooted plants is necessary also...Flourish tabs work.
Pick fish on several criteria...they like the low pH, they have good color that catch the eye (schooling cardinal tetras for instance), they don't tear up planted plants (bushy nosed (sometimes sold as bristle nose) plecos as opposed to all others), eat algae (siamensis or otocinclus for instance) or eat plant pests (clown loaches love snails!).
Don't use an undergravel filter...the roots of many plants don't like the flow. You do need to keep the roots warm, though. If the tank is in a cold location and the water isn't circulating through the gravel, it will stunt growth. Serious FW plant keepers use heating mats to go under the tank...I've never used them, so I can't say whether they are worth the cost.
Decorate with dense driftwood. I've never liked what happens to most of the driftwood that the LFSs sell...it rots and grows hair algae like a farm. There is some African tree root stuff that I love. It has been several years so I don't know if they still have it but I found it at Exotic Aquatics in Boulder. The stuff is expensive but it holds up and doesn't float. I'm sure other stores carry it as well.
Create multiple levels in the substrate using the driftwood or rocks as a wall. Some plants need very deep (5-6") root systems while many others don't.
If you want it, I've got about three buckets of gravel (Flourite and some other that looks the same) that came out of the 50 that I'm converting to an anemone tank. You're welcome to it if you want to make the drive--it would handle 1/2 of a 125. It will need to washed thoroughly, but the price is right.
I'm sure there's more but that's about all that I can think of right now.
