Well I just took the plunge and bought one off eBay... (not even pg yet!)
I had been reluctant til now as they looked like they were shortcutting on safety stuff, like no spring cover, and the foot panel looked about half as high as the Amby one... but the one I just found had all the things I required and to me looks totally comparable to the Amby... for $100 including postage. Even if the mattress seems less than ideal, I can buy a new mattress from Amby that would fit as the dimensions are the same...
And I can get sheets too - so yay!