RE: Dev environment for custom solutions only
Hi Peter,
Any full Dynamics 365 Customer Engagement license will allow for customization. There are different components/modules OOB in the Sales and Customer Service modules so that is a consideration. Microsoft is not going ahead with the business edition version of Customer Engagement, so it is only the Enterprise license.
As I mentioned before, you could sign up for a trial. The trials at the demos site are for 90 days, I think the regular trials are also this long but it's been a while since I signed up.
Your best option for licensing would be to join the MPN and purchase an Action pack subscription ($475/yr). It includes Dynamics CE licenses internal use. Alternatively you could look at the current SMB promo pricing (Approx $75/month). You would need to purchase this from a partner.
Hope this helps.