I can give you a few guidelines though and a few links as well.
If you get an e-bike direct from China you need to be somewhat handy and knowledgeable about bikes in general because assembly is usually rather haphazard even if the parts are decent.
I would definitely go with a 36V motor-battery setup, 24V systems just don't have the grunt that the higher voltage units do (all else being equal).
It wouldn't surprise me if a front motor would be better for your situation (ice, snow, studded tires) because you will be pedaling the rear wheel while the front is motorized which gives you two wheel drive. I also think in your situation that 26" wheels might be better than 20", larger wheels would tend to work better in low traction situation than smaller ones. The last link to Ebay has a 26" wheel folding e-bike, reading the entire listing on that bike will tell you more than I have time to relate here and the text seems to be fairly accurate, particularly about most of the e-bikes for sale being inexpensive machines with an e-kit installed that are selling for two or three times the cost in parts.
Prodeco seems to make some of the nicer affordable e-bikes out there and since they are a US company communications/parts/service are at least possible, in my experience Chinese companies are really bad at those things but I'm very handy and don't need as much support as the great majority of people.
If you have any more questions I'll be happy to try and answer.
http://www.sportinggoodscentral.com/products/view.aspx?sku=9917354&af=1727&cse=1727&gclid=CMCSkJi19LUCFQo3nAodyAgApw
http://www.westmarine.com/webapp/wcs/stores/servlet/Product_11151_10001_1024708_-1?ci_src=14110944&ci_sku=1024708&cid=sc_googlepla&gclid=CIj43oa79LUCFQnNnAodE2UAvA#.UT2yx9a-o6w
http://www.westmarine.com/webapp/wcs/stores/servlet/Product_11151_10001_1288650_-1?cid=chanintel_google&ci_src=14110944&ci_sku=1288650#.UT2zUta-o6w
http://www.ebay.com/itm/Prodeco-Technologies-2012-STRIDE-36V-250W-6A-LiFEPO4-Electric-Bicycle-Bike-eBike-/271075265757#vi-content
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