Giorgos, just to ask, why did you sell your old system? Since the title of this thread touches on better quality being a must, sadly any step up in quality is usually comes at a price. If money is tight for you, now may not be the right time to invest in higher quality equipment.
I usually suggest buying the tops you need and then adding enough subs to keep up and give you the sound/volume you desire, but as others have mentioned, for your application the subs are just as important. I agree that the 835s are the best choice for tops you've listed so far, but will also require more subs than you can afford for the system not to be heavily lopsided - you'd probably end up wanting 4 818s for 2 835s as a minimum.
The 812 over 818 combination would work as a smaller option compared to the 835, but you'd have to decide if the 812 is sufficient enough for your work as compared to the 835. A pair of single 18" subs will still only go so far...
Honestly though, if you can't afford to buy the system "right" out of the gate, rent what you need until you can. I understand that rentals are very difficult in your location, but a piecemealed lopsided sound system isn't going to do you any good even if the individual components are higher quality than what you originally had. Good luck!
