If I purchase a template from videohive and edit it for my client can I use the end-video (encoded video) as my portfolio?
no, definitely not. copyright issue.
You can showcase your ability to edit templates for clients, but you should make this crystal clear that all you did was the editing of the template and not the design of the template/animation.
If you just place this in your portfolio, you are suggesting that you created a work that you have not created. Which obviously, you should not do if you care about your reputation.