On the Overnight Sensations. They are good speakers but they are not a good "value." You'll spend about $200+ to build them all in all and at the end of the day that money could have been spent on a better pair of used speakers that still have some resale value. I've built 5 pair of different DIY speakers (Overnight Sensations included) along with my own design from Seas drivers (my avatar). Only my own design was a good value proposition as I got the drivers for free. On the low end DIY is not a money saver nor do the speakers wipe the floor with all others in their price range. Take the OSs for example. My build ran $250. My Q Acoustics 2010i cost $129. The 2010i are absolutely the better speaker to my ears though it does not play as low as the OSs.
Just some food for thought. I'm not knocking DIY at all. It is a great hobby and over the years I was very active in the local DIY community. I have heard some amazing designs that won't break the bank; Pluto and LX Mini come to mind as do some Fostex designs. The best system I have heard was actually DIY. A close friend of mine has some OBs designed by Nelson Pass that run a Lowther and 15" Eminence woofer.
Anyway, have fun but if you're looking to save money DIY is not all it is cracked up to be unless you go higher end; a design costing a grand in parts will usually outperform a thousand dollar retail speaker. BTW, the enclosure is really where you can beat out the commercial stuff. Brace the hell out of it, line it, etc as you don't have to worry about shipping weight and such