If you do go for the expansions, I'd recommend getting the two Border sets (Klingon and Romulan) and the consolidated rulebook. After that, you can add some of the (______) Attack sets (Romulan, Orion, Hydran, etc) and/or some of the booster packs. The (_____) Attacks sets add new ship data cards and counters as well as some new map panels and a rules/scenario book that covers whatever it is that the expansion is focusing on. If you bought the consolidated rulebook, all of the rules will already be in there but the scenarios won't be. The booster packs add new counters and ship data cards.
You can also get .pdf versions of ship cards and rulebooks at DriverthruRPG and download free ship cards and scenarios at the link I provided earlier.