I need to build transaction on machine with internet via go sdk and then sign it on offline machine
There is no need for the internet to build the transaction. Only you need to network to submit the transaction to horizon. To build the transaction you can use go SDK and need to export as base64. at the moment as i know we can get the base64 XDR after signed only. So if you want to use multisign then you can sign and build the transaction in go SDK and send the XDR to others and get their signs as well. They can sign through laboratory or even you can provide any client interface where you use one of the stellar SDK and sign and return the XDR. To make the transaction succeed you need to have the internet connection and you need to submit that to stellar horizon.