I didn't receive a PM, so I guess you posted directly. That's okay, navigating the forum can be a steep learning curve. You'll get the hang of it. The best advice I can give is to not let your initial enthusiasm get the better of you. Take the time now to do your research and planning. There is so much valuable information and hard lessons learned here on the forums that you really can not afford not to soak it all in.
I would stay away from the ISO. My understanding is that it is less friendly to work with (crumbly and more fragile). I'm not familiar with the specifics of what glues are compatible with it, but seem to recall having read that it "is special".
Pink, blue or even green (Menards?) XPS is what I would stick to (pun intended). I have had success with TB2, GG, GS, PLP and epoxy. Each has their advantages and disadvantages in a given application. Regardless of the glue, best results are always achieved when the parts fit together properly and are held securely while the glue cures.
Regarding keeping a 5x8 cabin under 300 lbs, you are probably talking to the wrong guy! With all of the extra wood and features in TPCE, using hybrid construction, I will blow that number away!
You are going to want to keep things very simple and use as little wood as possible. Look at GPW's small foamie (now Bonnie's) and Eagles Mickey Mouse build. You will need to have no or very basic electrical; think head lamps and battery op puck lights with no on board system. You will also most likely have to make your own doors (tho I couldn't say what factory doors weigh, I bet they weigh more than mine... schmaybe).