I sugggest you to buy at least 3 filters to used them individually or together, according to the light conditions.
I have the 0.3 (ND2) 0.6 (ND4) and 0.9 (ND8) which reduce the light of 1-2-3 stops. I think that using at least 3 filters is the only way to achieve the right ND value in different situations, I can't see how it would be possible with just 1 or 2 filters. Obviously, to use more filters at once, you'll need a matte box (even a little cheap screw-in one) or an adapter, like the Cokin P. The Matte Box is better because it works also as a lens hood, that with the Cokin P adapter you won't be able to mount.
Trouble is, to use 2-3 filters together you have to buy really good filters, otherwise you'll end up with strong color shift (usually towards magenta) and sharpness reduction. A cheap alternative to HQ glass filters are the resin filters, some of them are quite good, but you definitely can't get a good 6 stops reduction with them, not even using a single filter. That's just my experience.
I once tried a 250 $ "slim fader", an ND filter that works, I guess, something like a polarizer and produces from 1 to 9 stops reduction, very easy to find on eBay. It was really bad, when you reached a 2-3 stops reduction you could clearly se a sort of dark cross which splitted the frame in four areas. Nice special effect, but...
On ebay you can also find complete ND and gradual ND filters set (6 filters + 1 filter holder (like Cokin P) + 8 screw lens adapter from 49 to 82 diameter) for less than 50 $. I never used them because the filters are too small for a MatteBox, but I'd be really curious to try.
To know which ND value would be good for you, just do the math: if you're shooting at 1/125 f/8 and you wanna shoot at 1/50 f/4-2,8 you need a 3-4 stops reduction. So, in my case, I'd use an ND8 or an ND8 + an ND2. That's all.