Grads ND2 & ND4 to start of with.

The two used together will be equivalent of an ND8  Most times you will be able to push the graduation down enough over the lens to use it as an ND filter ( use one upside down if necessary ).  Depends on the lens of course. The P series is not suitable for wide angle lenses. 
I wouldn't invest in any others just yet see how you get on with these. 

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html