I am new in this field so this question might look very simple to
many. I have an E1 connection, that connects to a 'Converter', which
is connected to a Cisco 2501 router.
I have been thinking of getting a hardware firewall as our network is
always under attacks and current software firewall and proxy server is
of no good at the moment.

Is there any product, of any company that is a firewall and router
with built-in T1/E1 convertor?
I need cost effective solution so if anyone can recommend brands or
more than one company, it'll be better. My network size is 75 nodes
but soon we'll expand to 150, so I need a solution that is sufficient
for more years.