Right, hang on, because there's a lot to unpick here. Essentially, there are three 'lines' of Dredd books:
- Rebellion has had a somewhat ad-hoc Dredd line of reprints that started back in 2005. These include the four items you linked to, and titles still in print are listed on the
2000 AD shop.
- The Judge Dredd Complete Case Files is a Rebellion series that started reprinting all of the Dredd tales in 2000 AD – and then also the Magazine – in broadly chronological order. The US series is some way behind the UK one, but the contents are identical, bar covers. The first America tale was omitted from the CCFs for reasons that, frankly, don't make a great deal of sense. The line switches to (part) colour with volume 13. Note that to also get strips that appeared in annuals and specials, you'll also need the Restricted Files (four books).
-
The Judge Dredd Mega Collection is a hardback partwork by Hachette, which reprints selected tales from Dredd and the wider world (so Judge Anderson, Devlin Waugh), etc. It's done in collaboration with Rebellion (2000 AD editor Matt Smith collated the collection and provides intros to each issue; PR guru Michael Molcher adds interviews and essays), but is not Rebellion product.
Naturally, there is a ton of crossover between these series.
If you want everything Dredd, your best bet is to continue with the CCFs. Note that the series (assuming sales allow it to continue) will eventually catch up with and include the likes of Origins and Chaos. If you only want selected items and/or don't have the patience to wait for the CCFs to catch up, grab the Rebellion modern Dredd collections that appeal to you. The JDMC is more for hardback themed collections that sit somewhere between the two.
Also,
Barney should be able to assist with what's covered where.