Answer to first part, you'd need to be out a few times a week, firstly for confidence and secondly it's a bloody long paddle. No don't get a racing boat, your mamba would be fine if a little slow. Racing boat takes practice that you need to be doing to get confortable with just doing the distance.
I did from the portage to the get out as sweep a couple of years ago and all i can say is don't do it if you leave the bung open on your boat. Also after about 30 rescues and no training I was fucked falling out of the boat.