Designing a Half-Suspended Shelf
The wooden support beam ("ledger" around here) is nice because it supports the shelf weight evenly. It also gives you the opportunity to put a lot of fasteners in the wall if you feel like a few isn't going to be good enough. The bad thing is that there's relatively little to anchor against a horizontal pull forward. So, if it was me, I'd combine the ledger with a smallish (10cm/leg) L bracket on each end to prevent that pull. (For aesthetics, I'd put the bracket on the top side of the shelf.
Sag-wise, in all likelihood, 4 support points will be adequate. If your load causes sagging, simply add a metal or wooden strip. If you see sagging on the short edge, add wood or metal in that direction.
If your wires are anchored well into the ceiling (note, this is tough, as a straight pull out -- "tension", as opposed to "shear" -- is the weakest direction for a concrete anchor), this should hold anything short of a book and bullion collection. Be sure to use turnbuckles or equivalent to ensure that your cables are carrying the load equally. I'd suggest when you shop for anchors that you aim for something like a 10mm x 75mm in an expansion sleeve design. For reference, this would be something like an ITW Red Head Trubolt Wedge Anchor, which offers around 3000 pounds resistance to pull out. To attach wire to a bolt in the ceiling, use a "bolt hanger".




If you set the anchors and test load them and discover that they aren't holding well enough, you might put a (roughly) 100mmx50mm wooden ledger on the ceiling. This will allow more fasteners into the ceiling and permit screw-in eyebolts for the cables.

One last note, if you're drilling into a ceiling that's not entirely owned by you, check to see that you're not going to accidentally drill into water, power, or (worst case!) a post-tensioned slab.