By external res, I mean something you can cool directly that's preferably outside the tent. If you have all your water inside the tent, its ALL getting warmed by the lights, and you'll end up having a setup outside the tent to use a chiller anyway(if its dwc). I dont recommend running nute solution through a chiller, and to avoid that you need to have a plain-water reservoir to cool and use that to transfer heat out of the nute solution.
If you go DTW, E&F or even E&G buckets you can just use ice bottles in a reservoir under a tray or outside the tent when you need to drop the temp a few degrees. Cant do that in DWC as the ice will touch the roots and thats no good.
If you plan to use a 250w, you may not even need to worry about water temps, just get a strong enough fan to pull the heat out. And to answer your filter question, yes, you'll probably want a 6" fan, 4-500cfm to go with a filter and a 250-600w light.
You may just want to look at a 4x4 tent and a 400 or 600w. I'm not into talking people up in size for no good reason, but if youre already considering it, you're probably just going to expand after a harvest or 2 anyway. The slightly larger equipment you'll need is a pretty negligible cost too, bigger you go the cheaper it gets. Just my 2c on that.