There are quite a few help documents that will assist you.
1)
Quote:
if they do not pay WooCommerce plan for their account, does my client site disappears from the internet?
No, the site will still be active including themes. Any other licensed products/plug-ins will however be disabled -
Woocommerce FAQ[
^] and on Wordpress the site will still be available but only on the 'Basic (free) plan' -
Manage Purchases, Renewals & Cancellations[
^]
2)
Quote:
What else should my client ask from the previous developers to take full control for his website?
This will all depend on what their requirements are regarding the new site, do they want all of the current features and/or plug-ins etc.
3)
Quote:
Can I transfer complete website to another WordPress account and how?
For Woocommerce, yes you can -
Can I transfer a subscription to a client, co-worker or friend?[
^]
For Wordpress, the same answer, yes you can -
Your WordPress.com plan and any other paid upgrades will be transferred to the new owner and will remain with the site.[
^]
If you have no other choice than to use Wordpress, you have to do with what you get. If I had a choice and depending on the clients budget, I will start from new and creaet their site from scratch.