As you can see... there's a lot of options open for getting started in combat.

The basic SPAD Derilict is capable of being used as a trainer... so the transition after soloing wouldn't be too bad. Its also a very inexpensive, durable plane.

The Gremlin flying wing.... wait a bit. The hardest part about transitioning to that is keeping orientation. suddenly not having the fuselage makes the first few flights "interresting." I wouldn't advise that plane until you've flown in 3 or 4 combat contests. ( You can fly in combat as soon after soloing as you're willing to take the risk... Mid-airs happen.)