Here is an abridged answer I just gave to an architectural academic who is interested in using games and game modding and AI (bots) in consideration was something like 3DVIDIA/Virtools, Unity or Unreal.
I hope this is of use to others or if my sourced information is out of date or inaccurate please comment below the post!
I am no AI expert although I too have an AI project I wish to develop: I don’t know 3DVIDIA, I do know Unreal (UT) used to have AI research projects, I don’t know if UDK has easily available AI projects but UDK does look good.
Overall Unity is probably the easiest and their asset store has many content packs and scripts and avatars for purchase (including for AI) and it runs on Mac and PC and quite well on older machines.
You may not need the Professional version, I am not sure.
An introductory summary of difference for level design: http://www.worldofleveldesign.com/categories/level_design_tutorials/what-level-editor-game-engine-should-you-use-how-to-choose.php
A game engine comparison is here: http://fragileearthstudios.com/2011/10/24/comparing-cryengine-and-unreal-and-unity-too/
For heritage settings I have been told UDK (http://www.worldofleveldesign.com/categories/udk/ue3-vs-udk-vs-ut3.php) is very good but I have yet to use it.
Of interest to you: there are tutorials for UDK bots
UDK AI Director:
Free tool: http://www.moddb.com/forum/thread/create-bot-ai-with-pogamut-in-udk-ut2004-ue2-or-defcon
UNITY AI research
Wandering AI http://answers.unity3d.com/questions/432027/wandering-ai.html
Robot AI from Mecanim example http://forum.unity3d.com/threads/166633-Mecanim-quot-AI-quot-Free-Sources
AI applications (basic overview) http://unitygems.com/ai-applications/
Tutorials in Unity and Basic AI http://devsbuild.it/resources/type/article/unity-basic-artificial-intelligence-part-1
A side project might be to review the new SIMS4 and Sims Societies.
On a more urban design and cinematographic level, one used to be able to link Simcities and the SIMS2 avatars but I have not heard of related urban design or BIM research.
We are also going to look into Skyrim the Crysis/CryTek engine, which may be too powerful / complex for your needs.
I personally will also look into, the earlier version, Oblivion, had very simple scripting tools for avatar interaction and also had good modding tools (but it was not multiplayer, at least not officially).
Skyrim (Creation kit) has built in habits for NPCs, which I believe can be modded/affected, new buildings/settings can be added relatively easily.
For ease of use, most designers and researchers seem to be using Unity, but UDK (free download for PC) may have some benefits if you find similar AI projects.
If you want to show behaviours and don’t mind or can replace medieval settings, Skyrim may suffice.
For path finding UDK may be good, Unity can do all of it but if you can’t find prebuilt software in the Unity story, you may need to build it from scratch.