Buff it with a clean dry towel. A wet towel will just smear it around. It should come right off
The wait time on most grouts is 3 weeks to fully cure but IMO 2 weeks is enough time.
BTW... expensive sealer and cheap sealer = the same thing. The only difference is you may have to apply the cheaper stuff 2 times. If you get the cheaper stuff, apply it, let it dry, drip some water on it. If it beads then youre all set, if it soaks up, you need another application.