Have a think about SSL - the whole point of it is to prevent a man in the middle attack - to make sure the end point you are really talking to is who they say they are.
Some systems that insert themselves into an SSL conversation do so by generating their own private CA certificate. You then have to load this CA certificate onto every machine that passes through it. Every machine that does not have it gets a security warning in their web browser or application.
Apart from that method, there is no other way of doing transparent inline SSL caching.
The next closet thing you could consider doing is setting up your own content node. Cisco have a solution called "Akamai Connect" that runs on ISR routers.
https://www.cisco.com/c/en/us/solutions/collateral/enterprise-networks/intelligent-wan-akamai/datash...
This only works when Akamai is used as the CDN. It usually goes inside your network. This allows you to have a private Akamai node. When uses request content from an Akamai CDN the DNS that gets returned is your local private node. This then retrieves the contents, caches it, and returns it to the user.
One bonus - it does allow the caching of Apple iOS updates ....
This wont help with You Tube or any other CDN.
You really need a lot of users for this to work well - who request similar content. I could see it costing $US15k since it sounds like your deployment is on the smaller side of things.
I don't know where you are in the world, but have you considered getting a Satellite connection? You'll probably be able to get something much faster and the pricing is not too bad these days. If it was me I would rather spend the money on a satellite connection.
