Yes, there will be mana pots. And many combinations gonna be used , since IL is really one of the finer chronicle when it comes to balance. Most common combinations would be Des/Tyr (expect some nerf), nuker/tanker (UD on robe armor will prolly be nerfed), dagger/tanker, and some classic combos like Sps/EE , HE/Wk, etc.
If you check dnet forums, you'll notice that many old clans or their members announced their presence already, and this news is just couple of days old.
Subclass system works like this: XX/XX + XX/XX Meaning if you start with let's say TH and you can stacksub it to any other class of SAME race (human). Along that you can do retail sub and let's say you take SPS for retail sub, you can sub SPS also but once again on the same race (elf).
Additional info - PVP zones around epic bosses. This could be really interesting.
There are C5 download links on forum, but still if your internet is somehow limited and you can't download it, I think that it is possible to play it trough Interlude client, because one guy actually succeeded in doing it. However you will still have to download server system file which for the beta was almost 300 MB , but I'm not sure how it's going to be for the live edition. Still, my recommendation would be to download C5 by any means, because you don't want some bug popping in your face when you 're least expecting it.
On topic, Dradden said that we can expect ETA today :)