Online CRM version Up gradation- 2016 to Dynamics 365

Hi,

We are planning to upgrade our online CRM 2016 to Dynamics 365. What are the things which need to be taken care of

e.g. 1) How to check all the java scripts working properly or not as there are more than 100 scripts

       2) How to check all the plugins working properly or not as there are more than 25 plugins

       3) How to check whether all the custom workflow are working properly or not as there are more than                      10 custom workflows are there.

       4)Apart from this we have more than 100000 Accounts, Leads, Opportunities etc.

Is there any code validation tools for javascripts, plugins?

Thanks

Amit