Upgrading Drupal

I'm putting together a quote to update a site from 4.6.5 to 4.7.2.

The site uses ecommerce.

Here's the list of activated modules:

cod, parcel, payment, paypal, product, shipping, store, tangible

Can anybody give me some sort of idea of the issues, or amount of
work that will be involved in updating the ecommerce side of the site?

Even just selecting from these options would be useful to me:

Trivial | Easy | Average | Significant | Don't bother quoting

Of course more details are also welcome.

How to add new input field in the content or node editable area
Example: Presently there are two fields Title and Body in this section, I need to add one or two more fields like description and keywords.

I am playing around with the update options to bring my site to the 4.7 breanch.
So far everything seems to work normal, except the update of the amazon module fails with this error message:

After upgrading from 4.6.2 to 4.7.2, there is no longer a "select all" button or checkbox on the "list" tab of "administer"->"content". Does anyone know how to fix this?

Are there any obvious places to look for the cause of failures in the 4.7 collapsing/expanding area enhancements? I'm working with a legacy custom template setup (that is NOT a simple one), if that helps.

Ajax stuff also doesn't appear to be working properly. The update.php file reported its progress with text messages, not a progress bar. I have not yet enabled free tagging to see if that works or not.

I've just upgraded my mirrored test 4.6 site to 4.7, so I can work on it without breaking the production site.

However, as it's using Private file method, it's still using the the Private files from the production site, and when I attempt to change this in the Test site's settings page, it breaks every audio/image file on the Test site.

(Just to clarify - if I leave the 4.7 Test Site using the original Private files, the images etc are working)