[Exim] many questions

Hello,

I have set my exim mailer as a mail hub for my domain, which only forwards
mail from (or to) internal mail exchange (so, there are no local deliveries
at all).
It works great!
Now I want to add some features and I'm not sure all of them are possible.
I'm quite new with exim. So, if anyone can help with any of these, I'll be
very gratefull:

1. I want to limit message size going through exim to 300 kB (there must be
some faq for it?)
2. I want to filter messages according to their attachments, if there are no
attachments, or Content-type has doc, xls or txt it can go through.
Othervise, I'd have to reject the message (somewhat silly security policy).
3. I'd like to have backup system for internal mail exchange so if message
cannot be delvered to the first internal mail exchange, it should go to the
other. Must be some failback feature?
4. I'd like to create address translation for incoming mail such as change
to xx@??? to aa@??? . It would be easy if I had local deliveries, but
I don't.

Any halp would be apreciated.

Hana Breyer Priselac
System Administrator
PBZ
Zagreb, Croatia