Hi everyone, 

> Will I find a rather easy way of making a I beam, or is it more simple to just do 2 U beams that I glue together ?

The latter is easier, but it's a waste of material, because most of the material will be in the wrong location, strengthwise.

You are working with composites now, so other shapes that are difficult or even impossible with metals can be used.
Does the part really have to be shaped like an I beam? What about other shapes? E.g. a box structure?

XXXXXXXXXXXXX
XXXX          XXXX
      X          X
      X          X
XXXX          XXXX
XXXXXXXXXXXXX    (Picture is obviously sideweays, rotate by 90 degrees)

This can be made from 2 U shaped parts and 2 flat sheets glued together. Easier to manufacture and stronger than an I beam of the same weight.

> I saw tutorials on the internet where the guy puts his composites in the oven and some where he does not and I
> haven't yet figured out what the true difference is ?

Whether heat is necessary depends on 1. the resin used and 2. the required strength.
ALL resins benefit from elevated temperature curing, and some absolutely require it.
As a rule of thumb: resins with longer pot life benefit more from the heating than those with shorter pot life.
 
You don't need an industrial oven to do this, often you can get away with using a styrofoam box and a few lightbulbs, or a cardboard box and an electrical heater with fan.

> Can I avoid the infusion part (although I find it very cool) ?

Sure, hand layup still works fine. Vacuum bagging too.

My final advice, same I always give: start building the prototype with cheap glass.