Windows server and exchange 2012

Basically, i have to take over the maintainance of a Windows server environment, fairly new to this but have supported servers, AD and exchange admin in the past so not an issue but need solid guidance for which I am willing to pay for results so looking for skilled people who can help me through this. Its not been maintained at all over the last year not even windows updates, the exchange server fell over 3 months ago as one of its databases grew bigger than the 1TB windows exchange 2103 standard can handle but instead of being restored each user mailbox was recovered by using the cached data on each machine, seems to work but now flacky as constant problems such as calender permissions. Need to get the whole network in a good state, basic administrator job, look at backup being done correctly, clean up AD accounts of users left etc. and anything moving forward. At the moment desperate help with setting up an Human Resouses drive which has a security group giving all HR department members full access but need to given 3 none HR people access to one 'Holiday' excell sheet which they can modify users holiday and sick days but no access to anything on that drive, How do i do this using best practices? I have been given these guidelines