To be honest there is little tactical advantage to adding the 'tactical' gear to a marker.
The only benefit with a red dot sight is if you use it as a quick guide. Paintballs don't fire in a consistent straight line one after the other, and if you're looking down a sight then you're not keeping aware of everything else around you.
If you are into some of the special events we get to go on these days then lasers look pretty in the dark or in smoke - but point straight back at you, and tac-lights let you see in the dark. But don't put it on all of the time. It lets everyone see where you are are, or you sillouhette your team and give the opposition a good view. If you go to an enclosed CQB game then if there is darkness then there is usually also dust kicked up, such as in Drakelow tunnels. Put a light on and you can only see a few feet in front of you, but anyone else can see your light source.
A tac-light is invaluable in small rooms when searching for props in indoor scenarios.
The most useful thing you can mount is a camera, its probably better on your goggles as when on your marker you find a lot of ground footage, but mounting on a marker is the easiest setup and its always pointing where you are shooting.
EbairSoft is the cheap source of most addons.
http://ebairsoft.com/
I'm sponsored by ContourScotland so if you consider going for a tac-rail mounted camera I can give you a discount code with them
http://contourscotland.com/
