Is it possible to customize the script after purchasing?
Yes. Since the script is an open source project you can customize everything in the code.
After files customizations, can I continue to make updates?
No, after customizations you can no longer make upgrades (New updates will override your changes).
Does upgrade will erase all my changes?
New updates can override/erase your changes.
If not, can I customize a (front) view of my website without losing that during upgrade?
Yes, that is possible. You can do that by copying the existing view in the /extras/customizations/views/ folder by respecting the file path. Then, you can edit it. NOTE: This option is only available for the front (Not for the Admin panel views).
Can I customize my website's CSS style without losing that during upgrade?
Yes, that is possible. You can do that by adding your customized CSS code in the /public/css/custom.css file. NOTE: This option is only available for the front (Not for the Admin panel views).