Hardware and software suggestion

Dear Experts here, 

I need some help,

I have a project of Automation project to monitor and control an central air-conditioning system.

the system consists of  9 units  water chillers and 34 units AHU.

the 9 units chillers already has a PLC equipped with a modus  RTU module , the PLC model is

microtech III which specially for HVAC use.

For each AHU unit I will need : one analogue input, one analogue output, one digital output,

and 2 digital inputs.

The user should be able to read information from the chillers through the modbus RTU module and also to control the units and also read information from AHU and control them as well, 

All information should be viewed on a PC with an HMI software.

So assumed to use logo 8 for each AHU and initiate a network between all AHU units then to connect them to an S7-XXX as a server and central  unit and also to use this S7 -XXX as a master to control the other 9 units chillers as slaves.

does such idea is possible ? if so which CPU of  S7  is perfect to handle such project,

and which is the most stable HMI software.

Or else please let me know your suggestions.

Many Thanks.

        Hello nbashir,

        First of all why do you need a single controller for each AHU. You told about replacing logo 8 on each air handling unit. You can put a single main controller in the main panel and use remote I/O panels for each equipment. For example; a S7-315 2 PN/DP on the main panel and IM-151 3PNs in each remote panel. 

        You told that all the information must be seen on a PC with an HMI software, if it is a PC that turns into a scada application and you can prepare one with using WinCC. WinCC 7.3 is the older version but the one most used in applications. Also wincc professional in TIA portal can be used.