Covered in Grease? Cosmoline. I recommend a thorough break down and cleaning of the OP ROD chambre and gas piston tube. I have seen many SKS that have caked in Cosmoline in the Op rod area. Not sure if it dmages it or not but any increased resistance or pressure seems like a no-no in my book.
The firing pin recommendation is a sound and reasonable recommendation. AS you recall firing it you already know you won't get a surprise 10 shot burst......however prudent is the key. I have never presonally purchased a spring loaded FP but I have disassembled every firing pin group and cleaned meticulously. Carb cleaner is a good friend in cleaning. Other options are denatured alcohol and the Purple Grease cutter from Wally world. The grease cutter is interesting as it will turn cosmoline into a sludge of crap for easy removal. BE CAREFUL with the Grease cutter though.
Once thoroughly cleaned FP channel then ONE drop of CLP or similar....ever try Amsoil MP (metal Protectant)? I have enjoyed this product as a rival to WD 40 (water displacement is key)
Clean up like crazy and enjoy, WHAT A BARGAIN! $125?? I would buy 20 if available at that price.
Other accesories? I have few, a bolt on scope mount, SLING would be a good addition they used to be $5 for the SKS......thats about it. It is a straight forward fabulous design that is virtually impossible to break. If you want to get fancy you can send out for a trigger job.....but this is a battle rifle that was intended to shoot 3-4 inch groups. If you want a sniper, you should buy a sniper rifle
Enjoy it in its intended purpose.....have you ever offroaded in a Prius?