I'm a complete newb at this but I'm getting everything together now to start practicing and it isn't cheap so I want to get it right.

Anyone

If your mould is polished to a high gloss then you shouldn't need to do either.

Cheers mate 

Really depends on your longer term plans. If you are planning more CF parts then it may well be worth investing in the gelcoat gun. For a one off it would be cheaper to get your local bodyshop to give it a coat of 2k clear.
Parts made using epoxy resin will have no UV protection so something like a sunroof will fade over time. If you go the GC50 route then you will get a hard, highly gloss finish (assuming your mould is high gloss) with inbuilt UV protection that is chemically bonded to the epoxy. If you apply 2k clear after the part has cured then you will only get a mechanical bond with the epoxy.
Another advantage to the GC50 is that being a gelcoat once applied to the mould it will only dry to a tack, meaning that it acts like "spray tack" holding the first layer of cloth in place. Full cure happens once the epoxy is added.
Hope that helps.

Warren

Carbon Copies Ltd

I am planning on doing more parts it looks like I'm just going to bite the bullet and get a gun iv already got a compressor so it isn't that bad 

Because of the inner/outer skin the part should be naturally pretty stiff. I would probably go 2 x 200grm CF - 2 x 290grm woven E glass for both skins.

The gelcoat gun takes a bit of practice to get right. Worth having a practice on a sheet of glass, adjusting the air pressure and distance you have the gun from the surface until you can lay down a coat that is about 0.4mm thick. If it goes down too thick then you loose the clarity of the weave on the finished part. Because of the large nozzle it is easy to get big drips of gel on the surface. These will ruin the surface finish of the part. I find it best to start and stop spraying off the mould, this helps prevent the drips landing on the mould surface.
Again because of the large nozzle the gelcoat lays down with a slight "orange peel" finish, don't worry about it once infused the finish is like glass

Warren

Carbon Copies Ltd

Thanks for the help warren il post some pics up once I get started