Personally I would build my own sump inless you can get one cheap down there. I am sure there are lots of othe reefers there that could lend a hand. Are you planning on drilling your tank or using an overflow box?
For the sump there is lots of different designs. You just need to know what you plan on putting into it in the future. Usually a skimmer section, bubble traps, section of reactors/macro algea and return section. Not likely enough room for a refugium, but if theres a will...
If you want to run Ro/Di water then just bite the bullet and buy a system. If buying used you better check on how much it is to replace all the filter and membrane if there life is used up.