You can get reels like these
Various connector setups like :
http://www.olson.co.uk/iec309.htmMost of these would have "3 Core 2.5mm² Arctic Grade" cable
You might be best to ask for local advice in Belgium though as wiring regulations across europe while fairly harmonised are not absolutely identical.
Regs on industrial setups in particular can be fairly tight and US equipment can be way off standard.
I couldn't stress how important it would be to ensure that your customer gives you advice.
Can you supply the gear to the customer without connectors and with connection instructions?