There is very little difference between 2005,2006, and early 2007, however the later 2007 & all 2008 had a few minor upgrades. I don't know the exact time , but somewhere the rear brake was changed to a expanding shoe, whereas the earlier models had a band brake. The models with the band brake can be converted to the drum brake easily. The fact that Whizzer upgrades their product is one of the main reasons I like doing business with them, they just keep trying to make it better. Having said all that, All Whizzer NE models are great, and buying a new one out the door for $1500.00 is a good price, remember the dealer isn't going to get rich in the process. I don't know the tax breakdown, but I know the dealer paid a fair amount to have them shipped to him, so that price seems really good to me.
Most of the dealers, like myself, don't make a living by being a Whizzer dealer, but just enjoy having our own Whizzers, and sharing the fun with others. Back in the 40's, 50's, & 60's Whizzer dealers were also a variety, including bicycle shops, hardware stores, car dealers, auto repair shops, some Western Auto stores, catalog stores, Vacumm cleaner stores [I worked for Dayton Vacumm Sales & Service , a Whizzer dealer! in the early 60's], etc, etc. However it is a good system, because you can get parts & service at many different locations [your choice], and as time moves forward there will be even more dealers.
Hope this answers your questions,