Use silver based electric solder with LEDs instead of lead based solder. Use a small wattage soldering pencil, like a 15 watt one, so the chances of overheating the LED are minimized.
A dimmer is only useful if you like playing with a dawn to dusk effect, but has no value at all as far as plant growth goes. Much better, in my opinion, is a two position switch allowing you to choose a couple of currents, like 350 and 700 mA. Then you have a much better chance of getting a reasonable PAR for the plants.
is a nicely priced driver, and that site has others. Before deciding on which LED to use, do some math - if it is cheaper to use 16 - 1 watt LEDs than 6 high power LEDs, do so. The more LEDs you use, the more uniformly the tank will be lighted. Don't use optics unless that is the economical way to get the light spread you need, and it will very rarely be that.
Design the light so that it should give you more light than you want - you can always raise it a bit to lower the intensity, but trying to drive the LEDs to a higher current to get more light can lead to short LED life.
Better to use other than the latest and greatest stuff if the total cost is a concern to you. If you stick to 350 mA or close to that, you don't need to spend a bundle on a massive heatsink. Just a simple 1/8" thick aluminum plate, or an aluminum channel will work fine, at a much cheaper price.