Check out destinydatabase.com they have a bunch of stuff on the dlc. They talk about currencies and upgrading weapons. Even possibly from rare to exotic. It's pretty cool if they keep all of those possibilities. A good read anyway.

Dark bellow features hive enemies.. So solar shields and arc shields will be present (knights and wizards). So level the weapons with those attributes IMO. I think I read in the weekly update, but could have misinterpreted, but deej said something about revamping what you could buy with marks when the DLC hits, so could be new weapons, armour, anything! (Correct me if I did misinterpret that please!) As for who to rep farm? That ones all speculative, as no faction specifically has a "be all and end all armour/weapon" each faction plays to specific stats and player focuses.. So it's down to the individual!

I'm definitely slowing down my purchases in anticipation. Passed on Xur the past few weeks (although still kind of regret not getting Patience and Time) because I felt I wouldn't really use those weapons over my current exotics/Legendaries and didn't want to get back in the situations from early on where I wanted the item but didn't have enough coins for it. I figure there'll be some sweet stuff coming with the DLC that I'll be glad I saved my coins for. Less worried about the vanguard marks, since I there's not really anything left for me to spend them on. Just now working on crucible marks to replace the last few rares with Legenadaries from the factions. Does anyone think that the Vanguard (and factions) will add new gear that requires higher levels? I brought the factions up to Level 3 and then switched away to another one. My Vanguard is higher though. It would be cool if, say, Level 5 Vanguard gets access to more/better Legendaries.

I would save shards/energy for potential better weapons and armor pieces that are to come. I would also save your motes, coins and marks since we aren't sure what type of currency Eris will use.

Eris is supposed to be a vendor - if you look on the mobile app you will see a place for rep with her. There's to be an additional currency stream from what I've read as well. I generally end up saving my coins and rarely spend marks but maybe once a month. I generally try to max out my weapons when possible.