Minimum Rental Salary Requirement

I'm not sure there's any official requirement, so it's basically up to the landlords discretion.
The number that usually goes around would indeed be at least an income of 3 times the rent. The more ofc, the safer you'd be.
The landlord can ask you to provide proof of your monthly income so that he sees you'll be able to pay and still have at least a reasonable amount for daily expenses and what not.
Be aware though, that besides rent, sometimes there are additional fixed monthly costs like for cleaning (usually for apartments), or utilities. The listed rent might not always include those.
And often a deposit is also required, which is obviously returned when you happen to move out.

Thank you for your quick response!

That's brilliant. I am sure even with our fairly limited income we will get somewhere suitable.

Here's to a new life in Belgium!