I PREFER wood for the oven´s structure as absorves less heat than steel or aluminium and for me is easyer to make. Steel is also valid, but need to be well isulated.
4 x 100 W will probabilly be not enough. Try a domestic heater. I would suggest 2 x 1000 W. In case you need medium power, you could add a potentiometer to one of the 1000 Ws line. (Or better with electronic control !)
If you don't reach your requested temp, insist on isulation. seal edges and corners with mineral wool, fiberglass or similar. Increase wall thickness. recirculate your air and by last, increase heating power.
For the mold / plug.... if foam collapses on your test, I would suggest to make the mold epoxy wet layup room temp a few plyes.
This will create a sufficent strength to support next prepreg layers vacuumed + heated.
Once cured, maybe poscure it to resist prepreg´s curing temp. And then add the prepreg + high temp + vacuum as higher as possible.
Mold´s thickness.... I use to make about 6 mm for room temp wet layup. Made with glass. Up to 10 mm for big parts. If part is big, flexion under vacuum is more "able" (?). Less probable on small parts => thinner mold.
As the parts are not too big, you can use "thin" molds. Using carbon for the molds gives stifness.... I would go to about 5 or 6 mm.
Maybe 2 x 200 gr + 7 x 670 gr ? I would use gel coat to avoid fabrics's crosses marks on the surface. If you´ll make many parts, give a few extra plyes, to about 8mm as high temp + vac is a pain for molds (contraction, pressures, dilatation, on each part ....
I woul not use casting resin. Fabric gives dimensional stability, avoid torsions or other deformations...
That all is a fast recomendation... I´ve probabily missed any points...
Pay attention to details and think before making. Is not complicate !!