Think about, what the twisting in pairs is done for!

If the cable is hit by an interfering magnetic field, it intersects the gap between the two wires of twisted pair and a certain voltage is induced, producing a differential noise signal, which cannot be suppressed by the common mode rejection of trancievers, because the interference isn't common mode but differential. But as the pair is twisted, in the next turn, (nearly) the same noise signal is induced, but of opposite sign, resulting in a (nearly) total vanishing of noise.

This meachinsm does only work, if each pair is separately twisted. Such pairs are the A/B-signal of RS-485 loop, but also signal ground and an additional supply voltage. So, twist the RS-485 pair and the 18-24V and signal ground pair.

Twisting is not only needed to suppress the effects of external magnetic fields, but also internal fields, produced by the pairs itself! Assume you have to transport two RS-485 pair signals within the same cable. Further assume, that they can touch themselves, means that no internal shields are used. Then, each single wire can capacitively cross couple to the next wire it comes in contact with. But again, due to the twisting, with the next turn the wire of opposite polarity comes in contact to the wire and this capacitive coupling is of opposite sign! So, the capacitive cross coupling between pairs within the same cable is (nearly) totally vanishing!

Each pair of interest must be twisted, as close as possible and with as many turns per meter as possible.

About the shielding: Have a look at PROFIBUS specifications to see how they manage the shield: It's connected to protection earth, means chassis' grounds at both ends, so that the cable shields and enclosures form a complete Faraday cage.
Signal ground is isolated from the shield for DC signals. But there's a HF grounding at both ends, provided by 10nF caps (or so) from signal ground to cable shield.

Shielded Cat5 cable is a good compromise between costs and performance. But take note, characteristic impedance of Cat5 cable is 100Ohm and the cable is not specified for outdoor use.

Kai