For casual play, something as simple as a core set and a Y-wing and TIE advanced expansion can make for a solid game. Really, you should just buy what you want to play with. Perhaps one each of the original trilogy ships that you like the most to fit within your budget. You don't need much of a "purchasing strategy" unless you are aiming for competitive playI totally agree with this!
When I started, the first ships I bought after the original core set were (in order), Y-Wing, Tie Advanced, Millenium Falcon,and Lamda Shuttle. This gave me a lot of cinematic fun. I then bought another original core to add an extra X-wing and Tie fighters. I first though I'd be a casual player and add a ship or 2 a month.. Pfft... now 6 months in I am now an X-Wing crack addict! Besides the Huge ships I am only missing a couple.. and have multiples of many... and I go to local game store tournaments several times a month! It's a slippery slope!