If I can understand properly on eBay, all those heating elements are replacements for some kitchen oven models. That's the same thing as I was thinking to buy here. Find some reputable store that sell replacement parts for ovens and buy it. If that's correct I will avoid postage cost and waiting part. 

What is in the box with Omega CN7800? It would be of great help if you can tell me what SSR and what thermocouple to get since I really have 0 experience with those elements.

Just look for OEM replacement heater elements and oven fans on eBay. They are very cheap and very simple, so no need to try to shoehorn anything from another application that may not be suited to being in an oven.

Then CN7800 is on the shopping list  

What about heating device? Is it ok to use oven heater paired with fan? I would mount round heater with fan in the middle and something in front of fan so heat is distributed more even. 

As I understand now I need to get: PID with built in power supply, ssr, temperature sensor (type K?) fan, some kind of heater and heat resistant wires + oven housing materials

Ive got the CN7800, its a fantastic controller. Always dead accurate within 0.1 of a degree, and once you get a hang of the Chinglish instructions it is very simple and easy to operate.

Just make sure you get the model with the built in power supply for switching the SSR, don't do what I did and get the one with no power supply. If you do then you need to wire in a 9v battery or something like a phone charger to provide a load to switch the SSR. I can't remember which model is which, but have a look at the wiring diagrams in the manual and you can work it out. Or just ask Omega. 

Can someone please write me a list with all electric parts needed for oven? I understand that I need PID controller, silicone wiring for high temp, heating device and fan. 

I am considering Omega CN7500 or 7800, do I need something with it to function properly or it is ready to wire right out of box? Also does thermocouple comes with it or it needs to be purchased separately?

Thanks for sharing info with details. 
I will use high temp epoxy then instead PU 2K Varnish.

My plan was to finish and polish plug and after that I would use high temp gelcoat in 2 layers of aprox. 1mm thickness. On top of that I would add layers of epoxy compatible fiberglass and 2-3 layers of uni and twill carbon fiber 400 or 600g/m2. For all fabrics I plan to use high temperature epoxy as well.
I was thinking of 1mm gelcoat. Couple fiberglass layers, wooden or steel ribs for support, more fiberglass and at last carbon fiber layers. 

Mold will be pressed from inside out using negative pressure from vacuum bag on the outside with bags inside so it can form solid body from 2 part mold. 

Dentex, I recommended to you, don't experiment.
I have made a several MDF CNC machined plugs for autoclave prepreg moulds. There are a coulple ways how to do that efectively. I recomended to you, use only epoxy based coats - post cured, certainly not PUs. If the mould/plug is a single-purpose and hand laminate only, 2K-PS filler is ok.
If the plug/mould is a MDF massive block, the tested temperature resistance is 65C, for tooling prepregs enought. Alternatively, instead the tooling prepregs, is possible to use dry carbon cloth + infusion. In both cases, you have to post-cure the mould - out of plug. Very slowly going up to working temperature + 10C min. Remember that the post-curing process can ruin the mould because distorsion and cracks. Depends on mould shape the steel/AL support frame can help, especially for flat large moulds.
I have tested the MDF sheet  18mm thick and on 60C starts the distorsion, for massive block 150mm thickness on 80C seems a quite stable.
If the CNC available, start thinking about  epoxy tooling boards or AL machined moulds. That's the best, high accuracy, high effective way for prepreg moulds.

The oven heater plates or spirals are too massive pieces of steel. It means, has high thermal inertia. It means if the controller switches off on target temperature, in real the temperature goes up uncontrolled by tens of C. The heat is acumulated in the plate. That's the real problem for ON/OFF controller, not so for PID if is right tuned. Better to use the heat resistant wires, for example Kanthal based.
Good luck

Thanks for your answers. 

It's plug that is made from MDF not mold. My plan was to impregnate the material with 2K PU clear coat varnish, then spray pain it with epoxy primer and then spray it with 2K polyurethane, sand it and polish it. Can I use diluted high temperature epoxy instead varnish? 

Since plug is MDF, can I make mold with high temperature gelcoat and epoxy that cure on room temperature, release plug from mold after 2-3 days and then post cure mold without plug in it? I would use very slow curing cycle to reach desired temperatures.

My concern is that if I leave plug inside mold for curing up to 120°C, PU varnish could ruin mold or MDF could warp since it's wood and resin based.

Also, what kind of heater do you recommend instead of regular oven heaters?