Or could I still get a decent workout using the club step at 8 inches doing a Cathe workout that requires the high step ? Bearing in mind that, I only do step workouts at 4 inches due to knee trouble.
You could aggravate the problem if you get the step.
I had knee problems last year and couldn't take the high step above 8 inches. Now that the injury has finally resolved I proceed with caution. Just the odd workout a fortnight at 10" just to see how it goes is where I'm up to at present.
from where did you buy the step? I live in Europe too and have had difficulties finding someone who wants to ship the high step without charging even more than what you mention in your post.
I would choose option #3!: buy extra risers for your club step so you can get it up to 12" or so. Then, if you want to buy the high step later, just buy the topper and use the risers with it.
I personally like the high step better as it is easier to lug out the risers for only the step topper versus all the risers needed for the full club step. I have both and use the step topper on days wehn I am not using the full step (High step training, ME etc.)