I would definitely go with an internal return pump on a small tank. If you required a lot of flow (over 2000 gph), and/or you had a high lift to your display, then I'd recommend an external pump. With a standard height of 48" +/- and only needing approx. 600 gph return, an internal pump will certainly be the most cost effective and efficient pump.
IMO you'd need a bigger pump to run the return as well as a reactor. Make no mistake about it, Eheim makes a great pump. For your application it's just a bit too undersized.
I'd absolutely still go with an internal pump. You can consider either of these: