I already own New Vegas, but would it be cheaper to buy the Ultimate edition on ebay to get all the dlc's than it would be to buy them on the playstation store?

Dont know about ps3, but on pc is about 3$ cheeper to get ult. I in same boat. I just buy ult for convenience

No matter how you get the DLCs, on the PS3, just have two of them active per character, not session, but character from start to retirement. Otherwise the save files will grow too large and lag will happen over time.