900 is a good price for a van, and I suppose 600 for a guide isn't too horrible (they need to make a living), but IMO the great wall is really easy to do unguided (just download relevant content from the LP if you're curious about specific towers).

The idea of leaving luggage in random taxis used to freak me out, and I would still keep my passport on me, but if you think about it, you can get a great deal of info (e.g. car license, taxi license, phone number, wechat) on your taxi/driver before heading out on the hike. Furthermore, the driver will certainly want to collect his y600 from you, so driving off would be quite irrational. All that having been said, it is important to select drivers who appear trust worthy (gut instinct thing).