If you take the bike back to a department store for a replacement wheel (or replacement bike) the replacement is likely to be out of true like your current wheel. If you like the bike, your best bet is to take the bike to a bike shop and have the wheels trued. It should cost you $20 or less. If you are worried about the scratches on your wheel because of the rim rubbing against the brake, you should know that the marks will not be noticeable for long due to normal brake usage.