Ahjawad:
I applaud your effort to prepare a solution to something like this…You have to start somewhere and the Mikrotik products and forum are a great place to start.
Regarding your questions:
What Mikrotik products to consider for this project"
In general just using the Mikrotik Products are great for projects like this because of the flexibility of the boards and the OS. You don’t need seperate hardware for firewall, bandwidth management and security. It’s all there already.
You’ll definetely want to look in to Queue Trees and Hot Spot feature. Getting the bandwidth there is one thing…making sure there is enough to go around is another.
I’m wondering about the requirement that antennas can only be installed on the roof tops. If this is true, and some of the buildings are higher than 2 stories you’re going to have to come up with some creative ways to get service to the inside of the buildings. One of the ways to do this would be to give clients the ability to “Rent” preconfigured Routerboard that not only acts as a client but also as WDS/Mesh repeater for other people on the network.
The RB800 is ideal because you can get 4 high powered cards (e.g. R52hn) in them. They also have room for a compact flash card for caching and a CPU that is powerful enough to handle many clients. Although the RB4XX boards have 3 slots…you’re really only limited to 2 high powered cards because their isn’t enough room between the slots. The 493AH is a great board in situations where you need additional ethernet ports.
There are several new products coming out soon that will probably be of use to you as well…the are listed here.
http://www.mikrotik.com/download/share/hu11.pdf
The 750UP would be ideal for providing power to radios that you may have on a building. You could use it in combination with any router boards or the Omnitik. The new RB751U’s may be ideal for broadcasting inside of the buildings.
How do I create a ring between access points? ( I am planning to use sectors and APs)
and
How do I create mesh network for backbone?
In a project this large…you’re essentially going to need two networks - one to connect all of the repeaters together …and another to actually provide access to the clients. It’s not much different than what most everyone else here does except that in your case everything is squeezed together. Spend some time reading through the other posts…most of the recommendations still apply in your case.
Ultimately, you’ll have to do some in depth planning to come up with a configuration of antennas & Routerboards that make sense for the job. Ideally, you’ll setup everything so that you have enough 5 ghz channels to do both backhaul and clients (more and more hardware is supporting 5ghz so don’t just assume you can use them all for backhauls).
What are the best places to install the Sectors/antennas?
This question really can’t be answered without more information relating to the buildings, topography, power locations, local restrictions, etc. If your really wanting to do this project yourself…it may be helpful to bring someone in for a day or two to help you come up with a plan. You’ll most likely save money and time in the long run by starting off with a plan that not only works in the beginning but gives you options for expansion and speed down the road.
What are the points to consider for the project like this?
There are two many to list…but here are some of the big ones:
Budget (For installation and future maintenance)
Ease of Maintance (if every time you need to replace a piece of equipment it requires a bucket truck…that adds up…think about it ahead of time.)
Number of expected users
Expected bandwidth per user
Availability of power to APs
Emergency backup
Remote management (when considering budget, include remote poe reset devices like the 750up so you don’t have to reset devices at 4 in the AM)
Customer Support
and did I mention Budget 
Your right on track by starting here at the Mikrotik Forums. However, as the previous poster mentioned…it may be wise to hire someone to come in and
make recommendations.
You may be able to save in consulting fees by offering free room and board 
?
Good luck on your project - Hope that helps