To obtain the full range of results from your UV I suggest:
1 W per 10 gal of water in your system (sump included if aplicable)...
10 gph per 1W flowing through your UV unit (actual flow rate, not rated)...
Thus if you do not have a sump:
I suggest 30W or larger UV light...
Flowing at 300 gph or slightly less...
My favorite UV units are Danner's Pondmaster lights. They are built like tanks and are fully submersible...
Turbo Twist lights work great but are not built like tanks. So handle them with care.
I do not know of any manufacturer of UV lights that I would steer you (or anyone) away from... and just to mention, I hear good things abotu Aqua UV...