Welcome to AH2 hornet.
You dont need to "buy" the game. What you downloaded is the full version. You only have to pay a monthly fee (14.95 USD) to be able to use the online arenas after your two week trial period.
Yes, flight model needs some getting used to. Low speed stalling is normal, depending also on fuel and bomb loadout and plane type. What i want to say is: Flying a fully loaded P51D is a lot harder then a SpitfireMkIX with only 50% fuel and no bombs.
I recommend you take a look at the HELP file as well as
http://www.netaces.org . Both have a great deal of info on how to get started, stick setup and everything. If you run into any trouble, never hesitate to ask.
Use your two week online free trial to get some time with trainers in the Training Arena. They will help you with basic flying up to advanced fighting tactics.
I hope to see you up in the air soon

.
Schatzi