You can mod them all quite easily to add full android market, checkout
slatedroid.com. Ive done it myself with my Hannspree Hannspad. The tablet shipped with 2.2 and no market, I updated it to 3.0 with full market but have since settled on 2.3 with full market due to massively increased speed and due to a small bug on 3.0 which spoilt one of my main uses.
The real trick is identifying some of the cheaper ones as you need to know the chipset and board model to do any modding. Another thing to consider is that a lot of the cheaper ones only have a resistive screen which means no multi-touch lower sensitivity and a lot less accurate. You also lose some features in android without multitouch such as page zooming.