I would like to know if i have to use the box he provides or can i use any STB
You don't have to use it, if you want you can get it directly from SCV office, although it would be easier to buy from the local cable operator, if you did every month you could pay the tariff to your cable operator and he would pay it at the SCV's office.
can i use any STB with CAS
For an SCV connection, you can use only scv's STB, other STBs won't work.
with CAS and if possible there r any STB with DVR capability.
Not that I know off, the STB scv gives is very basic, it doesn't have any DVR function.