Your res will be fine, as will its draw for the pump, since I'm assuming there will be a straight shot of tubing from the res to the pump (use thickwall/reinforced tubing or you'll regret it). The res is just to bleed bubbles and store extra fluid ... as well as a convenient place to fill/drain your system. I like the design ... very efficient. It also exposes your system to less risk since there are less places inside of your box for it to fail. Consider putting the box UNDER the case, though, to eliminate the chance of a leak pouring down into your system (and to facilitate filling/draining it).
Very nice design (to both of you
... though I'd do something about the enema tube, DarkNeo).