Well this is a little over beginners level 
(sorry if i am posting in the wrong forum, mod feel free to move the post)
I dont´t need a howto guide but usefull hint´s, pointer and comments are always appreciated.
the labtop in location B needs to access a remote desktop on C5 in location A
is this possible??
That is definitely possible. Once you have the SSTP tunnel up and running, you just need to setup some static routes. The SSTP server needs to know that 192.168.50.0/24 is available over the tunnel and the SSTP client needs to know that 192.168.1.0/24 is available on the other side of the tunnel.
Once that is setup, the laptop on the server side should be able to ping the RDP host on the c5 server. As long as your firewalls are permitting remote desktop you should be able to connect.
In short, yes. Make sure you have proper routes, adjust firewall(s) if needed and enjoy happy end.
I assume that by default, SSTP client will have route to whole 192.168.1.0/24, so this direction will be covered. And you can add route to 192.168.50.0/24 to VPN account on server side, so that it can be automatically added when client connects.