Well try starting off in moderation. Start off with sketches alone to build up with those that would want something. If they want it lined/colored/etc. next month or batch they can ask for addition. Ooooor if full from the start. Only do sketching and leave a watermark over it so they can't just take the sketch and run. Then they pay before you do more.

Don't go full blown is what I'd say to start. Start off small and see how it works before you add more options.