I've been looking into this as well but haven't moved on it yet. Here are some companies I've found, though not fully explored yet. Let me know if any of these work out. I'm leaning toward working with a game engine but haven't had time to act on this yet.
http://digitalurban.blogspot.com/search ... %20Engines
Digitally Distributed Environments: Game Engines
http://www.turntool.com/
TurnTool
http://www.anark.com/
Anark: Enterprise Class 3d Interactive Solutions and Software
http://www.dxstudio.com/
DX Studio - 3D Editor & Real-time 3D Engine
http://www.hypercosm.com/
Hypercosm
http://www.insitusimulation.com/
In Situ Simulation
http://www.kaon.com/index.html
Kaon Interactive
http://www.lattice3d.com/
Lattice 3D software creates interactive 3D documents for technical and non-technical users
http://www.navisworks.com/
JetStream from NavisWorks -- View, Walkthrough, Coordinate, demonstrate and simulate 3D & 4D Models
(These guys are getting bought out by Autodesk... I hear that they can handle really complex models easily)
http://www.myvr-software.com/
myVR Software AS
http://www.simmetry3d.com/
Simmetry 3d
http://www.document3d.com/
QuadriSpace
http://www.quest3d.com/
Quest3D
http://www.cubicspace.com/
cubicspace.com
http://www.ventuz.com/
[VENTUZ] | Welcome
http://www.virtools.com/
Virtools, A Dassault Systèmes Company
http://www.vr4max.com/
VR4MAX - Real-time VR for Autodesk 3ds Max and Autodesk VIZ
http://www.walkinside.com/
Walkinside: 3D Walkinside ™ is the unique 3D CAD visualisation and simulation software, a product from VRcontext
http://www.michaelbrumm.com/genesis3dworldeditor2.html#
Genesis3D World Editor 2.0