I'm not really sure how he prices them, you can usually get a nicer mk II for cheaper than you can get an equally nice mk III, not sure which one you'd prefer, but I'd get a mk III unless you plan to have aftermarket sights fitted.
I've seen some crappy looking ones with double markings (Y and star of David) priced higher than ones with just one of those marks, I think some of them are priced based on the markings. I would just look at $450 and up, those ones all appear to be pretty nice.
As for the safety, the larger ambi safety is 100 times easier to use, I'm not a fan of the small safety, but that can be changed after the fact if you find that you prefer one over the other.
I would just take some time to look through a bunch and find a nice one. I went a little overboard with mine, I spent like 4 hours and looked through every single one that he had