The 100mbps will be just fine (according to others), however if you are using the campaign feature, or reporting lots of content on lots of accounts, it would be better to have the 1gbps. If you were purchase the 1gbps upgrade to to the 32GB server the price difference of the 64GB server is only $10 so you are better of getting the 64GB and get double the memory.
It will come with Linux for free, however MassPlanner will need Windows to run. So you would have to pay an additional $25/month to have Windows installed on it. If you send me a PM, I can tell you how to save the $25/month to make up for the cost.
Absolutely without a doubt!
I am running the 64GB with 2 installations of MassPlanner running 500 accounts each. I also have 48 other scripts running, and my CPU and RAM are under 60%