Hello krism;
Welcome to the forum; I hope we can get you information you can use for your projects.
From the nature of your questions, I believe you need to read up a bit on Profibus-DP networks and configuration. I suggest you look at
www.profibus.com (especially the Technology and products tabs); you can also download the profn2_e.pdf file from Siemens' support website: it discusses topology and material elements for the bus.
http://support.automation.siemens.com/WW/llisapi.dll?func=cslib.csinfo&changelang=true&aktprim=0&siteid=cseus&lang=en
These are the 2 best introductions I know on Profibus-DP.
Now:
1) basically, you should be able to use any Profibus-DP slave with any Profibus-DP master. As long as you have the gsd file (configuration datasheet) for the slave, you can make it available to the master and configure it as part of the network. Look it up in the links proposed above.
2) redundant HMI? that depends on the software used. Look it up in iFix help files; if it can handle redundant comm links, you will requiere 2 seperate Profibus-DP communication cards in the PC (such as Siemens' CP5611, or Softing processors... Look at the Products tab in profibus.com) and the appropriate software drivers.
3)Each DP master comes with some kind of Profibus Configurator, through which you integrate the slave gsd files, and set up the bus (slave node addresses, individual slave data exchange [size and format], and bus parameters [speed, length, number of repeaters...]). For S7-300, this is integrated in the hardware config editor of Step7. The configuration must be downloaded to the CPU before comms can be established, and slaves must have their own configuration established (node address, bus parameters equal to those of the master).
4) Profibus-DP operator panels are available from various manufacturers. Siemens has a very diverse line of them, programmed through WinCC flexible (formerly ProTool). There is a limit on the number of such "masters class 2" (as they are refered to in the Profibus world)that a CPU can handle. For the S7-300, that can vary from a maximum of 3 to a maximum of 8 (check catalog values for HMI connections for your specific CPU).
Remember that you can have up to 125 participants in the bus, including at lest one master, your HMIs and your I/O modules [this includes simple I/O, drives, transmitters, other CPUs...].
Hope this helps,
Daniel Chartier