No worries all questions are wlecome-
1&2) WHat you would gain in the base Essentials kit with vCenter is a central point to mange all hosts as the ability to maintain a set of gold images of VM templates that you will be able to deploy VMs without having to redo steps of installing an OS from scratch. Also with vCenter you will have more granuarity in the permission you can provide users which addresses your question 2
3) Yes having a single large datastore can be of concern -I would have split it up into smaller LUNs and be able to spread the load
4) I would take a look at the products by Veeam for backup
5) You might look around for older copis of vmware converter or it will be a purchase but look some of the vended tools like PlareSpin or Double Take for your migrations
