Can a group without big spending keep up?
Yes. No PvE entry in the game asks for anything you can only buy: dungeons are gated by level and Item Level, never by Combat Power (its entry field is empty for every dungeon). Progress is paced by Odyle Energy and by weekly kill counts that are the same for everyone. The Membership matters at the edges: it raises your Odyle Energy cap from 560 to 840, and you need it to buy on the Market. NCSOFT's membership notice also promises a second Odyle Energy injection when opening Expedition, Transcendence and Sanctuary cubes; the current client data does not show it yet (every cube takes one injection, with or without membership).