Hello,
Thank you for posting your query on Symantec community.
Here you will find in detail differences between Small Business on premises vs SEP on cloud.
http://symantechelp.com/CS?locale=EN_US&vid=v102071047_v97203175&ProdId=SEPSBE2013&context=sepsbe2013
Q. Can we get all the existing antivirus features in cloud model (SBE 2013)?
--> No, few features are not yet available. But later releases should cover them.
Can we block hardware devices unlike in SEP on prem
--> No, but can block USB access.
Can we install antivirus agents on Virtual Servers?
--> Yes
Can we use SBE 2013 in more than 250 computers as I have observed it is applicable for 250 users?
--> Yes, it's applicable for 250 users only but if there are more than 250 computers then Enterprise Edition is recommended version