1) Large Includes 200K of includes? Piece of cake! Remember, it is the server that is doing all of the work, not your clients. The issue is how many requests can your server simultaneously process and is there enough overhead to do it. With respect to speed, your server will cache the files after their first and repeated use, so you won't have to pull them from disk every single time.
2)Long Processes. 3)Reporting. Neither one of these things is a problem with PHP. The pdf library suggestion is a great one for reports. However, if it were me I would write a secondary app in my language of choice, like VB, foxpro, java, (but not access) which would still give me direct access to my data but would allow for some of the advanced admin/reporting tasks that PHP just didn't do well. Now, if everything must be web based, than you are limited to java or php or vbscript or other web langauges. 4)Loosely Typed Variables. mysql CAN BE very specific in it's typing requirements. PHP is a bit loose, but frankly html is looser. As the developer you have control to be as stricht as possible and validate/format all data any way you want. PHP has the tools to do this. me. 5.) But If there's a better framework for developing such an app, please let me know. Maybe there is, maybe there isn't. PHP will certainly drop the development time significantly. Give your requirements, I see no reason why you shouldn't move forward.