Believe it or not the value of the old, plain Challenger start at $200 in parts condition, upto over $1000. in new condition. From what I can see(not real clear or complete pics) it looks to be in about NRA "Good" condition. This puts it in the $400 range.
Very worth the money and excellent in quality. Another John Browning design and made by Colt.
You could single load any standard 22.BB cap, CB cap, Short, Long, Long Rifle as long as it is chambered for the Long Rifle. It may not be though so look for the chamber stamp. In the very old days they could be chambered in Short, Long or Long Rifle.
Just make sure to thoroughly clean the lead/powder residue after each shooting. And check the bore before you shoot it the first time for old possible damage or excess corrosion. A local gunsmith is recommended to also check the more delicate parts like springs, extractor etc to make sure it will be safe to use.
Nice find. Grips are out there on sites like Ebay.
Get yourself another magazine. There are repros out there but nothing is like factory. Just have to shop the used p[ adds, gun shows.
Here is a pic of my Colt Match Target Woodsman. Their most deluxe model.