Acceptable price : As with anything it depends what you're willing to pay. I've know people sell them for a £100 (don't touch, likely to be hot) or get people who want one so much they pay more than a new one. The 3G is more desireable that the WiFi as you can use it anywhere, although I just have Personal Hotspot activated on my iPhone so never use the 3G on mine.
I've had the 1,2 & now the 3. My laptop is almost redundant. I still have a heavy duty base unit for other work, but most day to day stuff I use the iPad for.
A Wifi one cannot get blocked, a 3G one can, but they can only block the 3G functionality of it.
iPad 1 is noticeably slower than the iPad 2, the 2 has cameras front and rear and graphics are superior. You can also use FaceTime on the iPad 2
Issues, just check that the screen is in good order, it responds to touch, no cracks, it's not lifting out of the casing. You "could" take along a lttle auroscope if you're fussy and check to see if the water damage indicaters (middle of the charging socket, bottom of the headphone jack) have been activated (it's been wet).
If you want one locally I know of a certain shop that has one in (3G & WiFi 16GB) for £275 and I know they'll do it for £250 for Wiki members