Wild Weasel is right. $150 is a good price, I'd buy it and I already have a few Norincos. It probably is a paratrooper model. They were made by Norinco for the civilian market and not issued to the Chinese military BUT some have scope rails on the left side and in that case its definitely worth more than $150. Finding the correct mount would be like finding the winning lottery numbers but ya never know.
Adding a high capacity magazine unless originally configured to accept them is illegal.
One benefit over an AK is the folding bayonet found on an SKS. Plus stripper clips can be loaded in an SKS just as fast if not faster than changing a magazine. Both rifles have high tolerance to dirt and can still function when dirty although the AK wins hands down in this dept. One drawback is keeping the SKS bolt clean to keep the firing pin moving free. They are more accurate than AKs with the Chinese being some of the better ones and the Yugos coming in a close second, at least from my experiences. Plus they're half the price of an AK.
Good reading material can be found here:
https://www.northcapepubs.com/
half way down is the listing for SKS Carbines. Other accessories, if not already included, are a sling, stripper clips, cleaning/tool kit (which is sometimes found in the butt stock) and thats about it. Avoid the receiver cover scope mounts as they rarely hold zero and any gimicky add ons are usually just decoration. Only time I'd change the stock is if its a Norinco with the rear sling swivel on the side of the butt stock. These tend to dig into your cheek when shooting if you're a righty.
As for diferences between Yugos and Chinese, the Chinese models were built closely to Russian specs while the Yugos had some minor diferences in machining and fit. Bayonets are diferent where the Chinese has a spike and the Yugo a Russian style blade, but most Chinese and Russian parts are interchangable hence the Sino Soviet models which are hybrids of SKS rifles made with both countrie's parts.
Almost anything you ever wanted to know about SKS carbines can be found here:
http://www.simonov.net/