Congrats on your find. Like everyone has said, oil it everywhere the manual shows to oil. Clean and polish anywhere you can get to easily. Cleaning under the needle area where often there is copious amounts of lint is a great idea. I've never messed with a 15-125, but it should be about the same as a 15-91. Once you've cleaned and oiled everything, sew with it and listen to it. If everything moves easily and quietly, I'd quit there and enjoy it.
You will find a few videos on the 15-125 on youtube, but many more on the 15-91 telling how to clean, oil and refurbish. The 15-125 is a 15-91 in a new body. The insides should be about the same.
bkay