I have a Chinese Norinco SKS with a serial number and blade bayonet that indicates it was made in 1964. It has no add-ons and is as issued with the exception of a buffer cushion under the dust cover.
It functions flawlessly with its standard 10 round internal magazine. Most of what Iíve read about the rifle is that external magazines introduce a significant un-reliability factor into the equation.
The price you mentioned seems high to me. But the modifications if done correctly and work without malfunctions may make it a good buy in todayís inflated market. Particularly when you look back at your purchase in a few years.
I have included a few very useful links if you want to increase your SKS knowledge base.