Note: Play what you like... but if you want to optimize fun/competitiveness on a budget. This is how I would collect. This assumes that two friends just want to play. Go one faction if that's your thing.
Haha... buy one of each... but here's the order I'd get.
1. Core Set (Buy the titles for Corvette and Neb-B in secondary market. They should be cheap.)
2. Gladiator (Empire) (The infamous Demolisher. All-rounder. Learn to fly a brawler.)
3. Assault Frigate Mk 2 (Rebel) ("Guppy" - All-rounder.)
4. Rebel/Imperial Squadron Pack
5. ISD (Empire) (Iconic Star Wars)
6. Home One (Rebel) (Iconic Star Wars)
7. MC30 (Rebel) ("Shrimp"- comes with Ordnance Experts, Turbolaser Reroute Circuits and Assault Proton Torpedoes)
8. Rogues & Villains
9. Rebel II/Imperial II Squadron Pack
10. Flotilla (Gozanti/GR-75)
11. Corellain Conflict
12. Liberty/Interdictor/Corvette/Neb-B/Raider
13. Victory