Question-and-Answer Resource for the Building Energy Modeling Community
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IMO this is exactly where energy modeling shines, to answer questions like this, and I would absolutely use an energy model in this situation.
Using a piece of software like eQuest and the Design Development wizard, you can quickly investigate a number of different system types. You won't be able to examine more exotic systems like VRF or other complex designs that don't fit into eQuest's neat little packages, but at least you can check out the majority of the standard system types out there, as well as investigating various ECM's that may wish to implement. A skilled eQuest user could bang out wizard models in a couple of hours, or maybe half a day. A master of EnergyPlus could likely do the same, but my own (somewhat limited) experience suggests that E+ requires a little more finesse and detailed inputs, which you wouldn't have at this stage of the process.
You could also examine a free piece of software like RETScreen - http://www.retscreen.net/ - which will let you try out various pieces of equipment.
In this case, you aren't generating the model for a client, but to help your design process and give more informed answers to the client. Without a model, you'd have to fall back on rules of thumb, and as you stated in your question, there are so many complexities involved in a design that rules of thumb will probably just get you into trouble later on.
