I don't understand what you want to do.

What is the model going to be used for? That's the big question that has to be answered before anyone could advise you.

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NX8.0, Solidworks 2014, AutoCAD, Enovia V5

Used to show potential customers. This would not have to be super detailed. Would like it to be something you can navigate through so they feel like they are there.

Other models I would like to actually use to create engineering drawings from. Maybe we can make the first from the second?

I have heard Revit is popular now. I see AutoCAD plant 3d and those products too which seem nice... Just not sure which program so start learning if one makes more sense.

It's pretty important that you clearly define the end-goal before you start modeling, or you may end up investing in a model that you decide isn't worth maintaining, and start over from near-scratch, because it wasn't created in a way that serves future goals.

You mention P&IDs but if you're just wanting to have a 'dog & pony show' model, those are not really necessary. If you want P&IDs, then you'll want to model much differently, to make sure automations and parametric features are maintained.

What industry do you work in? Using software that is most likely to be used among subcontractors is pretty important. I've never heard of anyone using Revit in an industrial setting. Revit is pretty much Architecturally driven, and derivatives of Revit Architecture only exist so that subs of the Architect (Structural, Electrical, Mechanical) can work with the model and produce their work.

I used Autocad (vanilla) for 6 years to design and model on-shore and off-shore oil/gas facilities. Few platforms, and mostly on-shore "mud plants". I used Plant 3D for a year and a half after that to do mud plants. We were never able to get PFDs or P&IDs to generate well from the model and just did them separately. On one project, we just did the PFDs and P&IDs from scratch in plain jane Autocad, but we had a couple interns who needed the work anyways, as well as the learning experience.

If you're just creating a demonstration of "hey look at this pretty stuff" to try and impress potential clients then I guess it doesn't matter /what/ it is? Just start throwing in some models in an arrangement, model in some foundations, equipment, piping, vessels, valving, pipe racks, platforms, and a building shell or something. Would they even care what the P&IDs look like? Would they know what they are looking at? What's to say you don't just throw some P&IDs from an old job onto a new title block and make it LOOK like a complete package? If it's just a 'demo' model, why waste time?

Do you have a current design job that you want to try doing differently? If so, then what are your deliverables, and does anyone have to use the CAD model after you who would have certain expectations?

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NX8.0, Solidworks 2014, AutoCAD, Enovia V5

I see what you're saying. I definitely have to think about this and do more research first. Definitely don't want to waste a bunch of time. Thanks

If you have an existing plant, you could hire a group to scan the physical plant in 3D and give you an AutoCAD compatible drawing with everything there.

Revit is an architectural tool used by architectural firms, so that program would be one to consider, however, you should look at some work examples before investing in this rather expensive program.

Worth noting: Revit is not really compatible with Autocad in any intelligent way. Yes, you can export stuff from Revit that can be opened in Autocad, just like most any CAD software, but it loses any intelligence.

If you are a company that does it all in-house then this is of no concern (possibly) but if you are a firm that collaborates with other firms, or bids on projects and you must simply work with whatever they give you, pigeon-holing yourself is unwise.

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NX8.0, Solidworks 2014, AutoCAD, Enovia V5