What I did was look at all the different types of toy haulers out there first. I was originally going to go with a TT type but our 2007 GMC 1500 Sierra would be iffy to tow it so I decided I would get the a bigger truck. So we looked at what a SWR type of truck would haul and looked at that size... picked the 5th wheel TH and also the truck at the same time to make sure they both matched on what could be hauled.
Best thing to ask yourself is what do you WANT - we had ours built and looking back I might have gone with the Autolevel vs some of the little extras I got (no need for central vac or rear awning. But these are things you learn.
Good luck!
PS then you also need to look at short bed/long bed and whether or not you need a slider or not. That's a whole other debate!