I agree with Neuro and others to head towards a quality system. All ND filters have some colour cast, and Cokin has the most. I personally use HiTech (Formatt) and Lee.
You asked about hard and soft edge filters. It really depends on what you are shooting. If it has a definite straight separation I will use a hard edge. If it doesn't, say like a mountain range, I'll lean towards a soft edge. I also find I'm using the 2 and 3 stop filters the most, but I shoot sunrise and sunset landscapes. I rarely use the 1-stop versions.
An anecdote on a holder or hand-holding. I was shooting a seascape a few weeks back and decided to change lenses. In doing that, I dropped and lost my adapter ring.
So I left the UV filter on the lens and used chewing gum to hold the filter in place for my 30 second exposure. I wouldn't recommend it as a permanent strategy, but I got my shots. I now carry a small ball of blue-tak in my kit just in case. The holder is best solution, but you can survive without it.
So my recommendation would be to get a 2 stop hard and/or a 2 stop soft edge Lee or Hi-Tech filter in a 4inch (100mm) wide system and worry about the holder at a later date.
Hope this helps.