All of ours have took about a couple of weeks. It depends on what skills/knowledge you already have and what design/weapon your going with. Some feathers have been built in a few days some have taken years!

Have you seen the Zeobots? http://www.technobots.co.uk/contents/en-uk/d608.htmlhttp://www.technobots.co.uk/contents/en-uk/d608.html i dont know what your budget is, but these seem similar to your original ideas, then with any extra weight you can bolt more armour on.

alan
http://www.blazerbotics.co.ukwww.blazerbotics.co.uk