1. You will get lifetime updates of the theme.
2. Classipress is designed mainly as a classifieds site platform while Vantage is a business directory theme. Depending on your additional requirements, the themes will may require further code customization.
3. The difference between the Standard and Developer editions is just the PSD design files. All themes packages are not encrypted and so you can definitely modify them according to your requirements.
Yes, if you upgrade to the next level license package you will only pay the difference.
Please help our moderating team work more efficiently by not sending us support questions via PM. You can read more about how AppThemes support works
here. However, you can send a PM to follow up and remind me if I missed your support request/thread.
Thank you and have a nice day.