Walaikum Assalam. Allah mubarak karay apke liye bro. Do share some pictures if you can : )
it's better to get it checked by a mechanic (you can visit Carigar Restomods if you live near Johar Town, Lahore..he is a pretty much straight forward and knowledgeable guy).. especially get all it's fluids (engine, gear oil) changed.
you can install rear camera with the navigation system. it would work perfectly fine.
changing speakers is a relative term, if you feel the sound is okay, keep them, otherwise change them (i am no expert in sound systems though) : ) .