I can have a go at some of your questions for you:

2) The drill you linked is thoroughly reviewed by one of the forum regulars here

3) I am using Overlander batteries in the FW I am building and have used them in other applications in the past and never had any issues with them. I don't know if anyone else has used them in combat but I would imagine that some people must have. Looking at the quality of their packs I see no reason why they wouldn't be suitable. Also note that if you join the FRA then you can get a discount from battery supplier/sponsor Optimax.

4) Barrel Bolts are a good way of joining HDPE (and some other materials) together. Here is the first link that popped up on ebay for example

6) No idea what Homebase and B&Q stock regarding wire, but avoid the stuff intended for home/diy use with low strand count. I like the silicone insulated wire typically used in RC applications as it is very flexible, so easy to run, and the insulation is good for high temperatures.

7) Connectors to make it easy to test, prototype, upgrade parts, swap out damaged parts etc.

Good Luck!