Hey there, welcome to the forum!

Sounds like a good start, just getting your robot to survive a 3 minute bout is an achievement in of itself (which I confess not a lot my robots are able to do but we don't talk about that.. :P).

Yes to the HDPE definitely but wood does still work if used right. The Honey Badger, even to this day, uses wood as part of the main chassis and has held up OK though the BW variant definitely does better. Get the basics right first and then add a weapon methinks.

Soldering is one of those things that I personally have a love/hate relationship with. When it works, it works but I've been known to spend an hour soldering one XT90 plug into a wire because it just won't take so I wish you with that. Connections are quite easy, I am sure I made a diagram a while ago showing how that looks.