Consolidating servers with a focus on AD on different versions of Windows Server

I have two WS2003 servers virtualised with VMWare running as redundant AD masters for 2 other W2003 servers and 2 WS2008 servers and one WS2012 server.  Aside from file and print, accounting and web apps also serviced plus backup partitions distributed across RAID5 arrays on each machine.

There are regular AD errors but the network is operational although servers now need to be upgraded so new hardware and moreso new Windows Server software available. I suspect local resellers will not go below 2012.

My question is how to go about the process of evaluating the current system, whether rebuilding the AD is necessary (some older PC's might not reconnect well but this is not the deciding factor), and if so under which version to standardise, noting that 2016 has more virtualisation features but may be more problematic for older version sof windows clients to connect to ?

Any pointers, things to watch out for, greatly appreciated...