Your link is faulty.
I suspect that you are talking about collet extensions. A collet extender that fits in to an existing collet.
My understanding is that they are fine for routers mounted under tables, but should never be used for freehand use.
On reflection, I am not sure if you are still talking about routers that will be table compatible.
If you are simply talking about the router itself, I would expect a 1/2" to be supplied with a 1/4 collet. 8mm is normally an additional purchase.
If I were you, at that price range, I would go with the Trend rather than some no name chinese product. It should be compatible with most (underside mounted) router tables. Trend is a decent brand.