loving the mx5 mate.
heavy yes, not sure exactly but I'd say you wouldn't get much change, if any, from 1800kg.
cooler, zorst, nistune should see 200rwkw depending on boost. internally these motors will handle ~280-300rwkw with a metal headgasket and headstuds (proven in my other car)
I'm relativly new to stags myself but a dayz front bar from my favorite wrecker set me back $500. although if you hunt, parts can be found on the forums or eBay. but as you can imagine these arnt the most common cars so colour of parts might not always be what you after.
diff wise, shafts are smaller compared to gtr, but there is a thread floating around for a gtr diff swap.I've found mixed opinions on pulling the front shafts out. I haven't done it myself so I can't tell you from personal experience.
should be able to find more info searching.
hope this helps, i can't post links as I'm on a iPhone.