Unfortunately I ordered my materials already and I cannot order primer and hi gloss system from easy composites. Shipping would cost me the same or even more than products for those.

As I can see and understand, primer is thick polyester resin I guess with some fillers as well as hi gloss. Can I simply use basic polyester resin? 
You also mentioned 2k coating system, primer + topcoat. Would that be something like anti scratch 2k clear coat car varnish? They are polyurethane based usually and are easy to sand and polish to high gloss. 

I would apply by the brush but I dont mind sanding it and taking step by step.

Polyester resin would be easier to sand than epoxy, but it also wont polish up nicely. It's a compromise, end of the day there's lots of stuff you CAN use... but the end result won't be as nice as if you had used the correct stuff.

For the 2k, yes, any 2k paint will be good. You dont want 'anti-scratch', these will be formulated to resist abrasion - sanding and polishing is a form of abrasion. But any regular 2 component automotive paint will be fine. 

Also wouldn't recommend applying it by brush. Anything that is formulated for spraying won't brush well, and that is especially true of polyurethanes as if they are applied too thick they will foam and trap a load of bubbles. 

You may have to use epoxy if you can't get anything else, but it won't be nice to refinish - especially if you have fine details and small grooves.

I just actually need to seal the pattern and get like from 600 to 1000 grit finish to ensure good surface that is smooth enough for mould to separate.

Finished product will be for sure top coated with UV stable varnish and polished probably so I don't need to be "mirror" after releasing from mould.