Fer godssakes don't mention a bike. You still need a round trip ticket to enter the country with your waiver, so why complicate things by mentioning anything about buying a motorcycle? Just tell them you're going to be touring around by rented car, bus or train for three months, then leaving. Bring evidence you can support yourself (credit and debit cards will suffice), and be prepared to describe whatever will convince them you'll reliably return home (job, school, family, responsibilities, ties, whatever), since even with a waiver they'll turn you back if they think you might work or over-stay.
Never volunteer information or offer more than they ask. Keep it simple. Complexity makes them suspicious. Your goal is to blend into the background, which consists of tourists who'd never even dream of renting or buying a motorcycle. In other words, be boring.
Hope that helps.