Mixte frames are popular, and you got a deal. From the photos it appears that there may be more dirt and old grease than rust. I've bought a couple of bikes that I initially thought were parts donors, only to discover that dirt and old grease are great frame protection. When you get back from your vacation, give it a good bath and see what you have. If the paint is dull, I've had good luck with DuPont
#7 auto polish sold at any auto parts store. Caution - I've had two Nishikis, older than yours, and the decals were very fragile.