where to start is to go to the begenning of the DIY filter section and start reading every post with the word sump in the title. sure, it'll take a while, but when you're done you'll have a decent idea of how you want to go.
then figure how much you want to spend and how much effort you want to put into it. my sump is 60G and I spent $200 on just the media, but I expect it to last for a minimum of 10 years, and $20 a year doesn't seem as bad as $200 up front...
with your holes you can go with beananimal or durso style standpipes, from the pics it looks like they were running durso.
you can flow into one end of the sump and out the other, or into the middle and out both ends, you'll find examples of both in the posts.
cheapest/easiest would be pot scrubbies, and one pump. easiest but more expensive could be poret foam wedged into the tank as dividers, then you can start adding baffles and different media, multiple pumps, filter socks, etc.