I've got a 150g (72"x18"x28") that I'd like to build an LED light for but I'm wondering if it would be impractical on a tank that long and deep; especially for a complete novice with LEDs. The other option I'm considering is Buildmyled which will run a little over $400.
Depends on what kind of setup ur wanting. Is it going to be high tech, low tech, what kinds of plants do you have in mind? Do u have the dough to to spend on a pressurized co2 setup after getting a high light fixture.
If you don't mind not having the most efficient LED lighting, and can do simple soldering and wiring, you could make a light for that tank using the LEDs that come in tape form. See DIY SMD LED Pendant Light for details. But, if you buy a light, or some lights, you will have a more efficient system, using less electricity, and you can get lighting with good color rendition, so everything in the tank looks best.
IMO, a DIY LED setup is something that you have to want to do. It's a whole different hobby. I have done plenty DIY lighting projects to save some cash, make something fit a smaller tank, or making something more attractive but never LED's. I find LED's get overwhelming really quickly if you really just want an off the shelf fixture because even if you figure out exactly which LED's to use, then you have to decide if you want extra functionality and what it takes to do that.
I started planning my DIY LED build a while ago and by the time I had some idea of what I wanted, there were plenty of off the shelf units by that point.