1, What points should I consider for choosing best out of three apart from Cost and better functionality?
look for Certified solutions, as you mentioned both data & document archiving,
you can go here and find the partners
search with "archiving"
https://store.sap.com/sap/cp/ui/resources/store/html/Search.html?pcntry=IN&sap-language=EN&catID=&searchText=archiving&facetVal=&facetCode=&_cp_id=id-1452578205280-0
2. What would be your preference and why?
there are cloud solutions also available, instead of investing more dollars upfront.
this can be used to evaluate different existing solutions.
3. How complex it will be to move, already archived files from exchange directory to third party content servers?
this process most likely is not complex.
another way is to get quotes and product information brochures directly from vendors .
get these vendors to do a presentation , demos, proof of concept.
most likely they don't charge atleast initially for these
lastly SAP Content server could be also considered if the requirement is storing archive files and limited DMS