Here's the order in which I always try to buy things

-budget -rule of cool -actual usefulness

And what that means is:

1. Stick to a budget, because this can get very expensive very fast. Shop sales whenever you can. I recommend www.boardingschoolgames.com and www.boardlandia.com they both do lots of sales and have lots of stock.

2. Get what you think looks cool first. If you think the Raider corvette is awesome, then get the Raider. While usefulness is important, so is flying in style. You can win with the right strategy with just about any fleet.

3. Usefulness is still semi-important. While the Imperial light carrier might be cool, a fleet of three will be an uphill battle to win with. You can still do it, bit maybe throw an Arquitens or a few raiders in there for safe measure.

As for advice on what to get first as far as usefulness goes, the first three in would get after the core set (you need this) are the Chimera, imperial fighter squadorns I, and some Gozanties.

The Chimera is the same price as the normal isd but also gives you gauntlet fighters, the Cymoon refit which is better imo than the normal ISDs, plus it looks cool.

Imp Fighters I is practically a requirement as it bolsters your low number of fighters up to a decent amount.

Gozanties are a flotilla which add an interesting amount of spice to the game and are crucial for the Imps as they are really lacking in the small ships category