Will upgranding my MBP to an internal SSD, improve also speed of reading external non SSD HDrives?
Honestly, it is a waste not to have an SSD as an internal drive in a MacBook (or any computer in general), you will understand it once you have one. Of course, the speed of your internal drive has absolutely nothing to do with that of external ones. The latter is only governed by the respective bus speed they are connected to.