I agree about not getting the kid a pony!!! (Even though I think the remark was in jest.) Getting a teenage girl a horse or pony, even just as a lease, is like introducing them to crack. It's an addiction that they never outgrow. OTOH, being addicted to cycling is like being addicted to broccoli - people may look at you strangely but it won't ruin your life.
Treks typically group their models, with several models sharing the same basic frame but with different component levels. Get her one with a nice basic frame but the lower-end components. You can always upgrade it if she gets more serious.