I have a 500 combo that I bought several years ago. I paid $340 for a Mossberg 500, 24" rifled slug barrel and a 28" accuchoke barrel with 3 chokes, NIB. The 18.5 security barrel would cost $100 plus tax and shipping, NIB.
I would buy the 500 if it was in like new condition. At todays prices you can get the same setup NIB for $475. The 500 combo is $360 at walmart and the 18.5" security barrel is around $100. You might find a used barrel for less on Ebay.
I wouldn't give 50 cents for the winchester much less $500. Pistol grips ruin the accuracy of a shotgun. Winchester products are overpriced. 20 years ago Winchester pumps were one of the cheapest pumps on the market and they were better made than todays Winchester products. Not to mention a 20 ga is less than desirable for self defense.