Recommendations for a Gas central Heating system installer

We are looking for a Recommendations for a Gas central Heating system installer. Install needs to be done in Woluwe-saint-Pierre.

Our 30 years old Mazhout based system doesn't switch on more ('bruler' unit seems to trip when we switch on our heating system). Looks like it may cost around 1500 euro or more to change the tripping bruler unit. The worst part is , it is getting too cold , we have a tenant to answer and we still have 1500 litres of mazhout left in the tank.

There are three apartments for ~70 m2 each. This broken central heating ONLY does heating for winter across all apartments by circulating hot water
and energy consumption on individual apartments, is measured by calorimeters in each radiator. Each apartment has its own gas based heating system for hot water within the apartment.

Within 10 feet of central heating room, We have gas distribution for individual apartments on the basement through indivdual gas meters.
From here we can take new new gas connection from Sibelga for the new gas based central heating system if needed. If we need to draw another new distribution pipe, still not a big deal as steet distribtuin is just another 10 feet away.

The questions are:

(1)If re-installing, is it better to go for gas based or mazhout based central heating system ?
(2) Any recommendations for a brand of heating system ?
(3) Any recommendations for a installer who can install this in Woluwe-saint-Pierre?
(4) there are many types like condenser type etc, which kind of heating system to go for for energy conservation?
(5) Any ideas how much this upgrade might cost?
(6) is it better to take gas connection provision oursleves with sibelgas or let the installer organise this on our behalf?
(7) In case of upgarding to gas, any idea what to do with the leftover 1500 litres of mazhout?