Nope, you can just use resin. Tooling gelcoat is just more durable, so don't expect your mould surfaces to last very long. Tooling gels are also quite thixotropic, so it can cling to vertical surfaces. If you're just using laminating resin then it will run off vertical faces and pool at the bottom. For one off, flat parts, you can get away without it. 

I guess that you can answer your question yourself  

Speaking as a novice myself I would recommend you just pay the extra few quid (or dollars) and use gelcoat.
Working with gelcoat is a skill you need to learn too.

Yes. Your gelcoat will come out in the same condition as your plug surface. You can leave it and try to clean up the mould, the issue is that not only is it generally easier to sand the male (plug) version rather than the female (mould) version of the part, but gelcoats are by design extremely tough. Sanding them is a bitch. One can get rid of small defects on the mould surface by sanding and polishing, but to try to correct larger or more frequent imperfections is a tedious task indeed. You also run the risk of exposing microporosity bellow the surface of the mould, depending on the tooling system you use.

Thanks Hanaldo!