They are actually one in the same. There was never an orginial "model 98" only the custom series. The difference though would be found in the Tippmann 98 custom Platinum series with its easy to...
The best type of paintball for the 98 series is a mid grade paint (anything around 60 dollars like Diablo All Star). As for any gun, you don't want to just buy the cheapest paint possible (monster...
The best way to do it is to purchase a rail and attach the scope to that. Otherwise you can modify the scope to fit on the top of the marker using a hose clamp or other creative method.