To set up a new hull from scratch you will be looking at around $1200 or more for a racing monohull. Depending on what radio you use & if you use a modified or stock engine. You will be able to pick up a cheeper boat second hand, but steer clear of Ebay, few good deals on there. You will also need to decide what you want the boat for, playing around or racing. Also how big is the body of water you intend on using? What tye of hull do you want? Monohull, Catamaran, Outboard Tunnel, Hydro or Sport Hydro? All will have different costs associated with them.
As for 2.4GHz radios, Futaba FAST, Airtronics (Sanwa) FHSS and Nomadio are all proven over water. Spektrum has had issues in the past but have recently released their Marine spec receiver for use with some of their transmitters.
If you are after a monohull, another option for getting a boat set up is Retallick Engineering, Ken does a very nice monohull. there is also George Pap at Hobby Depot in Sydney.
I've dealt with all the companies Moofy07 has listed (except Wolfe Racing)as well as the 2 I listed & they are all good to do business with.
Hope this helps.