Hello there, I have some questions related to your ecommerce platform your answers would help deciding if yours is the right solution I can use:

1. During the integration process among your system and other templates, is there any problem to attach my own design to the current platform?
2. If I would like to use another payment gateway, for local bank in my country to easily collect the money, instead of Paypal or others, is it flexible to do that and replace the current one?
2. If bug appears on the system how will you fix it? And how will you provide me with the new one? 
3. Does your company name or logo appear on my website even web pages or scripts if I decided to have the open source? 
4. If I need to use the purchased version for another ecommerce especially I'm planning to add some modifications like other language (Arabic language which require rebuilding the system from scratch), should I purchase another version, or one for all? If not allowed can I pay for the license only and use the current version I already have? Your prompt response is highly appreciated.

1) You could use your own template design. But you have to follow the template architecture used by DotShoppingCart
2) Yes, payment gateway should be very easy to write. Check http://support.dotshoppingcart.com/forums/Home/tabid/36/forumid/6/threadid/76/scope/posts/Default.aspx.
3) Like any open source project the community will help you fix it. If you are using commercial edition, our company will fix that for you.
4) Yes, you are required to show DotShoppingCart powered by logo. You can find open source edition license at http://www.dotshoppingcart.com/Page/License.aspx
5) Open source edition is complete free. The commerical edition license is for one web site. Check the commerila edition license here http://www.dotshoppingcart.com/Page/DotShoppingCart%20Commercial%20LICENSE.aspx.

"You could use your own template design. But you have to follow the template architecture used by DotShoppingCart"

I like to design my own template also.  Is there more information or documentation on the template architecture used by DotShoppingCart?

Sorry, there isn't enough documentation about this topic. I hope I can fill these gaps after V2 is released. Check this thread http://support.dotshoppingcart.com/forums/Home/tabid/36/forumid/5/threadid/86/scope/posts/Default.aspx. Hope this could give you a start point.