Hi! I already had the code ready and it works exactly like your requirement: change the ssid after a preset interval (e.g. 15 minutes) and blink the led when the ssid get changes. The ssid = md5(MAC + random(min, max)). You can change min, max and the interval easily.
Please also notice that md5 result has 32 characters, however, there is a bug in the ESP8266WiFi library that prevent the ssid to have more than 31 characters. To workaround the bug, I use only first 31 characters of the md5 result to set the ssid. In case you need to have full 32 characters, I can also modify the library to fix that.
If you accept the bid, I can give you the code immediately (I assumed the delivery for this project is just only the source code & instruction how to run it). The code should work with any NodeMCU version.