Welcome to the board!
If you buy a Showcase body that's already cut for a Tele pickup at the heel, you can have them route it for a humbucker, then use a Tele pickguard with a single coil cutout to install a standard Tele pickup. Later on, if you you change your mind, you'll already have the route for the humbucker. You'll just need a different pickguard.
If you're not going to use a pickguard, you can always just route the cavity later. Templates for pickup cavities are usually in the $5 range. Find someone with a router and the appropriate bits, and cut a new hole.