Possible move to Belgium (where to live...)

Depends on your preferred living co diction. Rural village city neR transport driveable. May help if you say a little more

As a general point, immoweb.be is the most widely used site.

Can't make any specific suggestions without really knowing more, such as will you be working in the city centre or outside, (if you're working in the centre, you could consider commuting by train even from as far as Brussels) what sort of age group are you in (if you're "young", there is a student community in Mons for example, but it's not huge), what is important to you, a quiet life, or a social life and somewhere you can meet other xpats / international people etc. etc. What sort of budget do you have, etc. etc.

When we bought seven+ years ago, the immoweb site was absolutely invaluable. Among other things, you quickly learn which agencies overprice properties and which ones price spot-on.

It is important that you understand the purchasing arrangements here in Belgium which differ totally from those in the UK. While there are normally three stages to the process, it is the first, taking an option (offer), which is the most important because once an option has been accepted it is binding on both sides, within four months you will become the owner at the agreed price.

It is really important that you consult a notaire before you make even a verbal offer for any property, particularly if you will need a mortgage. While many notaires in (french-speaking) Wallonie cannot speak English, those who can speak Dutch (as well as French) often can. It is perfectly acceptable to ask an estate agent about the linguistic abilities of local notaires, they are not allowed to recommend any particular notaire but they can tell you which ones speak Dutch (and, if you lucky) English. A notaire's charges are related to the price you pay for the property (and the value of any mortgage) and their advice comes free.

It isn't the practice here to have a property surveyed, indeed the profession just doesn't exist, and it is your responsibility to look over a property for potential faults. If it looks as if it will need work done, ask a builder for an estimate.

In the unlikely event that a house turns out to be owned by a company and they want you to buy the company rather than the house, just walk away from it, the process is too difficult for a foreigner.

Generally, I would say that the house buying process in Belgium is much easier than it is in the UK.

Don't buy. Rent. At least at first.

Hainaut (the Mons region) is one of the most deprived areas in the EU.

Hmmm... Outside of Mons... Like a former pit-cottage in a village with 30% unemployment?

What puts you off Mons itself?

Mons has a population of around 100,000. Charleroi is about 35 kms away and twice as large. I doubt if you will get any recommendations/suggestions that you should live in Charleroi as it has a very poor reputation - a once major industrial city which has long fallen on hard times.
Anywhere else within a 30/40 km radius of Mons is going to be a small town/large village - 'surrounds cities' simply don't exist.
Your only alternative is to live much nearer to Brussels which is around 70 kms away but you then have a long commute and, the nearer you are to Brussels, the (much) higher your rent will be.
You don't say where you are from but, if you can, it might be worth taking a long weekend to get some sense of the place.

Mons is the eighth largest city/town in Belgium in terms of population.
Only Bruxelles/Brussel, Antwerpen, Charleroi, Liège, Brugge, Namur and Leuven are larger.

Of these only Bru. and Charleroi are close enough to be a possible commute, although the nicer areas of Bru. would be an awkward commute (as well as being considerably more expensive) and, as has been said, Charleroi was a major industrial that has fallen on hard times (although not quite another Detroit).

Mons was also once the centre of significant coal mining activity (the "Borinage") but has been much more successful in pulling itself into the 21st century. It was the European Capital of Culture last year.

Sorry, ninth largest, somehow Gent managed to get missed off the list.

The NATO base isn't in Mons, do look up its location. Mons is the nearest city to it. You're either talking about SHAPE 8km north-east of Mons or Chievres 20km north-west of Mons. SHAPE is 55km south-west of Brussels centre.