Linux since your using cpanel that is a breeze to move very easily.
However, the windows helm thats more of a headache unfortunately they dont have any migration tools for helm so that is going to be one huge headache.
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Sadly enough when selecting a reseller hosting provider you want to really pay attention to the pricing structure. The good ol saying remains for hosting that you get what you pay for. If you pay $3 for reseller hosting then thats what you will get so you will be looking on the high side for a good reseller provider. I would say that it is safe to say (judging by your requirements) that your monthly bill for that would be close to 20-30 per month. There are very good providers out there, searching for the best one for you is the tougest challenge. You may want to start by creating a list of atleast 3 that you have in mind. Once you have that done, do a search on WHT for reviews on the providers you have on your list. Perhaps you can check out some review sites like http://www.whreviews.com a very useful resource.