It all really depends on whether you're on a budget and want to have the fun of a bit of DIY, or want it as easy as possible.
The website you mention gets great reviews. They supply all the parts you need for your setup and also supply correx cut to size. The correx is also scored so all you have to do is build the grids, fold the correx and it'll all fit in place. Really easy and simple....but at a slight extra cost.
You can save up to 50% by shopping around (especially eBay) for the grids. You can regularly find good deals, getting lots of grids at cheaper prices, which you can build into various shapes and levels. The correx can be found online or at local sign making company. As long as your confident with a measuring tape and sharp knife, it's really simple to cut and score the correx.
I managed to build a 2 X 4 base with a 2 X 1 loft, plus ramp, for £47. And I've still got a large bit of correx left over.
As I say, it's all about what you want to do. Get the C&C pretty much ready made with correx all cut to size but pay a bit extra, or shop around, cut the base yourself and save a bit of money.
I'm Scottish, so I'll do anything to save 1p! Lol