1. GLA has good-quality products, period. There's not another retailer that I can say that about. And because of shipping and retail costs, you wouldn't save much if you purchased the same components individually. You could save a significant amount if you utilized Evilbay and possibly made some equal-quality substitutions, but you'd have to be confident in your ability to put something like that together, and/or be okay with screwing up once or several times and having to fix it. There also wouldn't be much of a warranty or customer support. Up to you. I've done it enough times that I could spend half as much for a higher-quality system, but that learning experience cost me a lot of money and a lot of time. Like, seriously, I get embarrassed when I even consider counting how much money I've put into this hobby, primarily involving CO2.
2. I'd suggest either a sump or one large - or, even better, two - reactor(s). A diffuser, inline or not, just isn't going to cut it.
One other thing - if you're new to CO2, high light will give you problems, and you'll spend the next several months trying to fix them. Medium light is a whole lot safer, and you can always increase it later. There's just not much margin of error when you go high. But, again, up to you. You'll learn a whole lot either way.
Lastly, I'd suggest a 20lb cylinder for that size tank. You can buy a standard steel industrial cylinder which will be easy to switch out whenever you need a refill. Or you can buy an aluminum cylinder, which looks a lot prettier, but is more difficult to refill because you can't just swap it out. Many CO2 shops don't do refills - only swaps - and costs vary wildly. So do your homework regarding local CO2 shops before deciding.