I think or offset is very common on all japan made trucks. Toyota/Nissan/Isuzu. He might be trying to sell you something more expensive. It would be hard to match the stock offset but you don't need to.
Baxman (see his gallery pictures at http://www.vehicross.info/gallery/sh...user=8&thumb=1) has 16" x 8" rims with zero offset and a 4" backspacing. That is very common rim size. Check out www.americanracing.com and almost every rim is offered in that size. The only difference is that your tires will stick out a little more giving a wider stance. Baxman has a bigger tire than 30" so his bumber had to be trimmed a little.
About the yokohama's, i don't know if they are all-terrain or not but you can get a closer look at them on a VX at this website:
http://homepage.mac.com/mrtew/VXpix.html
Or go to the yokohama website and see. They are much nicer than the stock 18" tires.
Hope this helps.