In the most recent issue of Field & Stream magazine there was an article about the "50 best guns ever made." The author lists what he believes were the best guns. Then at the end of the article he lists the 3 worst guns ever made. Listed as #3 was the Winchester 1400. He sites the gun was full of plastic internal parts that cracked and broke. I'm not familiar with this model, but if a writer that has been on a magazine staff for over 40 years, writing specifically about the quality and performance of guns, wrote so negatively about a model, I would avoid it real hard, especially since you say the condition is only 50%.
I am very familiar with the 870, though. I've had several over the decades, including a first year production 870AP that my dad purchased in 1950 and gave to me in 1959 on my 10th birthday. I'm still shooting that same gun and have replaced one 59 cent spring so far after 55 years of use. Today, after all those years of use the gun rates at 85+%.
My question would be, why do you want to purchase guns that would be in the worst condition when you received them? 50% and 60% are really at the bottom of the quality ratings. The blue book of gun values only goes down to 60% as the worst condition. Unless you just want to have the worst condition guns on your block, I'd pass. The 870 as you described it would be worth less than $100. If the serial number ends with a "M" then it is a magnum. You can put a slug barrel of the proper size and gauge on it and could fire 2 3/4 inch slugs, or 3 inch slugs if the barrel was marked for 3 inch shells.
Remington has made 2 different sizes of 20 gauge 870's. From 1950 to 1971 they made what has become known as the "standard" 20 gauge gun. It was made on the 12 gauge size receiver. It takes a 20 gauge barrel made just for that size receiver. It is a 20 gauge barrel that is stepped up to a thicker size at the breech end to fit in the 12 gauge size receiver. In 1972 they started making what is called the "lightweight" 20 gauge on a smaller receiver. The barrels are not interchangeable. If you want to add a slug barrel to your 20 gauge gun, you need to find out if it is the standard or lightweight model. Usually the lightweight model will have the letters "LW" on the receiver and barrel.
TennSquire