Most cheap filters have color casts, reduce contrast and sharpness and can vignette. Moreover, the glass is often non-toughened and can shatter into thousands of razor sharp shards that can scratch the front element. Toughened glass is not only, well tougher, but if there is a failure it tends to be a few big cracks that cause less issues. Another thing is the mount, good filters typically use brass metal which has a much lower tendency to cross threads, they are also tougher which makes knocks to the filter mount less of an issue. With cheap filters they tend to cross thread more often, and a knock to the mount can make the filter jam on to your lens semi-permanently.
These days there are a few companies buying good quality glass from the likes of Schneider which is the same as what B&W uses. Also polarisers film is now more more common and cheaper since it is used on LCD TVs.
With respect to the filer, why do you want an ND filter? Do you want a 2-3 stop or a 10 stop? They are very different things.
TBH, i would do something nice and fresh like capture sea with waves nice and sharp, not some overly repetitive long exposure blurry seascape. Hence forget the ND filter.
An ND-graduated filter on the other hand could be much more useful.
If you're not sure about spending £26-28 on the Camdiox filters then you can try these too: