For Immoweb, you ideally should know the post codes of where in Brussels you want to move to. There are 19 "communes" in Brussels region, they have I think 21 post codes in total (Brussels Ville has ?3 post codes, all the others have 1). There are areas just outside Brussels region, mainly in the south-east, where international families like to live, such as Tervuren, Kraainem, Wezembeek-Oppem.
I'd base where to live on where work is, what type of schooling you are looking at (international or local), whether you have a specific leaning towards living in Dutch or French or whether you prefer an English type of ghetto (that would be Tervuren, no offence meant).
You can get a lovely 4 bed house for as little as 1500 euro, even in the most expensive areas, even in the poorest and least desirable areas, you'd struggle to find a 4 bed house for less than 1000 euro.
There are certain places which are quite difficult for certain things like parking or local schooling, Ixelles immediately comes to mind when thinking of these things, so easy parking or a good choice of local schools with places might rule out certain areas to live immediately.
You can opt for apartment living, but it's far harder to compare prices as almost always there a additional charges on top of standard rent, sometimes it includes cleaning and maintenance of common areas, sometimes it includes hot water or even heating, so you always need to find what is included in charges and "remove" them when comparing like-for-like costs.
Contracts for housing are either fixed term or 9 years, 9 years are nearly always more desirable as you can break this type of contract, nearly impossible to break a fixed term contract.