A quick google search suggests the melting points of ABS and HIPS is above 140F, closer to 185-220. Still, if given a choice, I would find a cooler place to store the armor.
There are youtube videos by "dave's garage" of an FX kit assembly. I don't think we have videos here on the site, but Terry (Pandatrooper) put a lot of work into assembly tutorials on the main page: ----
I haven't seen a battery like that before, looks like it will provide good run time. Does it get hot? I run a 9V in series with 2 AAs to each fan in my helmet. Probably ends up weighing close to that one but they sit more flat.
Helmets are basically one size fits all. The angle of the faceplate during assembly can make the neck opening bigger or smaller by a little bit, but generally the amount and thickness of the padding you put in will size it for your head.
If you watch the movies you can hear the troopers clanking and clopping down the hallways, some plastic action is just part of the game. You may be able to bring the inside of the thighs closer to your legs and farther from each other with some good strapping though.