Merging models, or? Thread

I'm planning an RTS that involves customizable units. It "will" work so that the player choose different parts, put them together in a building designed for the robot construction and the robot is done. It's not anything fancier than the player choosing a head, a torso, two arms and a pair of legs, each part has different attributes, weapons and so on.

I'm sure moving attributes from one thing to another isn't a problem, but the 3D models and textures might be.

I'm trying to choose between the NeoAxis engine and the Unity engine to start working on whatever I can do, not much, but the problem I'm having is just that part that I don't know what would be the simpliest way for a programmer to make customizable units ingame. Would it be merging textures and 3D models? Keep every part separate and have one unit consisting of five parts? Any other method?

First one might be problematic ingame because of creation of new models and textures.

Second one might be problematic when it comes to the animation bones.

Gameplay from older game that serves as an inspiration.

Do you think both NeoAxis and Unity could handle both methods? Thoughts and comments? I would really appreciate input on this, I don't want to choose an engine, get some work done on it to later be told that what I want would be to complicated.