I think that pretty much any aftermarket exhaust execpt the X-can is going to be louder then stock.
As far as what to check, the normal stuff, but unless your in-law checked it I would pull the thing down and check the diff... everyone I have seen shipped from HPI was dry.
I have only bashed mine, but think I might try a race this summer, I am in the same area as Chris is. My car is still box stock execpt for the 8000 spring. I love to run it and run it very hard in my yard WOT is the norm for the thing....I live on 13ac, so space is not a problem.
As far as radios go I use the same 2.4 Futaba that I use for all my stuff, it has given me no problems.
A good place to order stuff from that takes paypal is DDM, they are tops, take paypal....I don't know if they ship to Oz or not tho.
Check out the
http://www.hpibajaforum.com great group of guys over there.