I'd have some concerns over security, I'd be looking at no less than a VPN tunnel between the two buildings even if you can somehow run your own cable.
It would be fibre for a start so getting up a ladder to a suspended run with a splicing kit is probably not so easy.
Neither is digging up a a channel to splice either.
Why do you assume getting physically onto the network would give you "Full access" to the network?
If they are going to do what you suggest they could also put a brick through a window and just plug straight into a switch port. There is being cautious and then there is paranoid. Adding another layer of complexity that can be thwarted by the most rudimentary methods seems like a pointless exercise.
I'd have some concerns over security, I'd be looking at no less than a VPN tunnel between the two buildings even if you can somehow run your own cable.
80 meters is pushing it for cat 5. If this is for a business you need to think about reliability and also the cost implications of the link failing. It's not just money spent resolving the problem, is money wasted because people can't work.
Get a few contractors in to give you some ideas. I suspect you'll be either looking at a fibre run or or sort of wifi or microwave link. Bare in mind they should also be able to use it for VOIP, meaning you could have a single phone system for both buildings.
I'd have some concerns over security, I'd be looking at no less than a VPN tunnel between the two buildings even if you can somehow run your own cable.
Pah, done a fair few installs over 100m...with absolutely no problems at all...