I've used Cokin P series for the last 10 years, I can recommend them. I haven't tried a cheaper alternative like the one you link to. The Cokin system and fitlers aren't particularly expensive in my mind, although I can't remember exactly how much they cost, but I wonder if it's not worth just getting proper Cokin ones. It's something you can build up over time, I now have lots of adapter rings for my lenses and quite a range of filters for astronomy and landscape use, but started off with only a couple.
In recent times I've bought my Cokin filters from
Digital Camera Warehouse
With modern digital there is a balance between using graduated filters and 3 shot HDR. I find the graduated filter is used less, but still has it's place. Perhaps worth considering if HDR can do what you want.