If you want to do the episodes, you can get through those with green gear. It will get easier with blue or purple gear, but lets say you want XII greens.
You can get through the episodes with the random drops you pick up along the way. If you really want to buy stuff, you can do all the episodes (and normal STF's) easily in white mk9 gear. Heck, I think I've only bought two or three mkX+ guns off anything, exchange or otherwise, for all of my characters in all of my 1.5 years of playing, and I don't even farm or anything.
And that's with me being a relatively unskilled player.