The 404 would be struggling with 2 RES in a 100 gal. I had a 404 in a 75 gal with 1 adult, and although it worked well for a while, it did get dirty a lot quicker than with the FX5.
My personal recommendation is the Fluval FX5 (I've got 2 of them). They'd be great for a 100 gallon. You can find them for about $250 on eBay. Two 404s would work as well, but then you have two filters to clean...
In general, eBay is the cheapest place. Sometimes you can find deals online from various dealers, though usually nothing is cheaper than eBay.
They're pretty easy to set up. The FX5 even has an instructional DVD that comes with it

They come with all the parts they need (hoses, input/output nozzles, clamps, etc.) but they usually don't come with any media besides the sponges. So you'll probably need to buy things like biomedia, carbon if you choose to use it, etc. But sometimes you'll get deals on eBay and the filter comes with some media (it's up to the seller though).
Hope I helped answer some of your Qs!