I can't help you as far as the file permisions go, but i BELIEVE (it may be wrong) that you can use pam restriction to help you do this over SMB.
If you have the latest version of samba, there will be an availiabe option in the config file "obey pam restrictions = yes" (or no). Pam restrictions, if i remember correctly, applys the local users restrictions to them.
for example, if you have user 'eman', and he has a home dir '/home/eman', and his home directory is such that he can write but not delete, then applying the pam settings would also do the same thing over samba. when he tries to access the share with that user name, he will be able to write in it, but not delete anything. I may be totally wrong and talking out of my ass, so wait for someone else to verify this before you try. i hope this helps.