Based solely on badge style (2nd generation 60s keystone with (R) symbol) and serial numbers of all 4 drums, it is solidly a 1965. And given other factors not visible in the photos, potentially Mar - Sep 1965.
Even if your sister is unable to clean it, it appears to be in very respectable condition & all original, and with the Downbeat configuration & Champagne Sparkle finish being very desirable, it should have excellent resale value.
Given this and comparing to some similar sold kits on Reverb with no cymbals and hardware (and some no snare), I would initially estimate it’s value closer to $2,000 - $2,500:
Reverb is a marketplace bringing together a wide-spanning community to buy, sell, and discuss all things music gear.
reverb.com
No snare...
Reverb is a marketplace bringing together a wide-spanning community to buy, sell, and discuss all things music gear.
reverb.com
Reverb is a marketplace bringing together a wide-spanning community to buy, sell, and discuss all things music gear.
reverb.com
No snare and non-original bass & tom mount...
Reverb is a marketplace bringing together a wide-spanning community to buy, sell, and discuss all things music gear.
reverb.com
Granted, it may take a bit to find the right buyer, but I would market the heck out of it...
— Take enticing photos... similar to Reverb ads above, and include any date stamps on shell interiors which can help validate a matching kit.
— Write a compelling description... one owner, all original, etc.
— List on multiple sites... Reverb, Facebook, VintageDrumForum.com, here, DrumForum.org, etc.
Potential buyers aware of the kit’s value won’t be deterred if only a little TLC is needed to bring it to it’s full potential.