Hi Sasane,
Please find my answer inline.
1) What are the questions that a client should be asked for choosing which version to use?
It's subjective so can not answer for you. Usually, you can start asking where they have their existing database, would they be okay hosting their marketing database in cloud etc.
2) Do all the users of ACC V7 could migrate to ACS? if not, what are the conditions?
It will be a manual process, there is nothing that automates this. ACS and ACC are two different ecosystems, Adobe is working on a connector to connect these two(it's working for some case but comes with limited features)
3) What are the other advantages of ACS in addition to being user-friendly?
It comes with integrated Dreamweaver so it's easy to build email template. You have built in dynamic reporting which is really nice.
Read below for more details.
https://helpx.adobe.com/legal/product-descriptions/campaign-standard.html
https://helpx.adobe.com/legal/product-descriptions/campaign.html
4) Can all the client use ACS even for touchy data ( bank, insurance....) ?
Again, it depends on the customer for example if they are okay in hosting their data over a third party network(Adobe) if not then ACS is not for them.
5) Which version is the most expensive?
ACC
Regards,
Amit