Are you kidding me, I don't know what kind of deals your finding! A 2008 Load Max gooseneck in any configuration up in this area for $3500 would be a steal. People think they are gold.
235/85-16 tires are cheap, and so is decking.
Weekendwarrior, lots of people flip the axles on their fifth wheel campers. Some people argue that the brakes on the axles do not work properly when reversed (flipping the axles). You should be alright flipping the axles in my opinion, you could always cut the perches and re weld them if your more comfortable with that option.
With any luck your coupler will be adjustable and with it at the correct height you won't need to flip anything.