Hello and welcome to RoF!
The Iron Cross Edition demo upgrade is a good place to start if you're going to be buying stuff for Rise of Flight. It gives you a nice selection of the most famous fighters to fly for a reasonable price. Beyond that, it depends greatly how you like to play RoF. If you like to play online, I recommend checking out this thread: http://riseofflight.com/Forum/viewtopic.php?f=49&t=26651&hilit=buying+guide
which may help you narrow down your choices based on performance and relative viability in online flying. For single player flying, I recommend choosing a squadron to campaign with and seeing which aircraft you'll need to play it. Alternately, you can purchase them as you go in your campaigning. Most squadrons don't use more than 3 or 4 types over the course of the war (and as a German pilot the free Alb D.Va serves in every fighter squadron at some point) so the cost is minimal. Keep in mind that you can also get additional things such as field mods, weapon mods and personalization stuff. The link above is a little out of date (it doesn't have the DH-4 in it) but it is very helpful in determining which field mods are useful. Also, there are scripted campaigns available and if you are interested in trying out the upcoming channel map, you can pre-order that too.