Well from one newcomer to another, welcome to the board! I Won't be able to answer everything, but I'll see what I can do.

200W seems to be alright for a motor, I know there are a few other builders who've made FW axe's that would know better. This link might help, it's a FW axe mechanism that was for sale. It's been sold, but it's got a lot of detail to learn from, such as shape and parts: http://www.fightingrobots.co.uk/thre...hlight=feather Also be aware that the axe motor is 24V while the cheapest Argos drills are 12V, so keep that in mind when deciding on power.

Lipos are bad when they go wrong, but in FW combat it seems to be rare: at least far less likely than a HW lipo catching fire. The benefits of a Lipo are too good to ignore provided you keep an eye on them: outside of combat, you'd see plenty of warning signs if a Lipo was about to go, such as the pack puffing up.

I believe that Ellis's way is the cheapest, but another alternative I found was Banebot's wheels. They're orderable online, and very useful for Beetles and Feathers. Jamie of RogueTwoRobots made an excellent video showing you how to fit them onto a bot:

So long as you have a grasp on what size the parts are that you want to use and know where you want them, the next step would be to just build a CAD of the design, work on it and/or show it off in a build diary thread to get feedback. Hope I've been able to help out.