Actually the size of your sump would have to depend on your bioload and type of filter media... if you are going to use cheap bioballs then you will need a lot of them as compared to eheim substrat or biohome, reason being that bioballs are less efficient. A general guide for sumps would be about half to 2 thirds your tanks capacity. I havent seen any of those fibreglass sumps on sale online before, only at those ponds and florist shops... so sry cant hlp u there. They come most of the time in blue or black and have kind of a rough surface. They also have drainage taps at the bottom of each compartment.
(theres gotta be someone around you that can help you out w. one, someone at a lfs possibly?