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Good news, Windows 2012 R2 is fully supported - see Server Products — Operating system compatibility
As far as all the different versions go, the full details are in the Server Products — Technical product specification but the main difference is in the number of onboard hard drive ports that run at 3Gbps or 6Gbps, and how many riser slots they have.
RPO and RPM have only one riser slot, and RPM is the only one with onboard video. They would go better in a rack configuration.
RPS and RPL have more riser slots, and would probably be better for having the extra room for multiple add-in cards. RPS has some advanced management options.
does that at least help you get started?