Is it possible to terminal login remotely (outside local network) to my mac at home via SSH + VPN?
Yes, you can use the iCloud Back to My Mac address to SSH/VNC in. You can obtain your address using the following command.

dns-sd -E


The final line shows the account number.
Use the following syntax for determining the address:

computername.number.members.btmm.icloud.com


Providing you have port forwarded correctly, you can use this address to connect over SSH/VNC.