if they can pass the 10' test... if they look prototype from 10' away (ie: no rivet counting), that's good enough for me.
i'm pretty well set with 1:8 scale tractors. i managed to pick up a few already. i've seen some R/C 1:8 scale construction vehicles (China imports), but i've yet to see anything but a promotional picture, and with the price under $50, i'm guessing they aren't actual prototype models but rather functional toys.
just thought i'd add this note. picked up a copy of "Scale Models Collectors Guide" from Abe's books for $7 including shipping. there were still 2-3 copies left at well under the $21.75 MSRP. one thing i can also comment on, ...book sellers/buyers seem to be VERY pick about condition. i've bought a number of books rated as low as 'good' and found them to be in great condition especially since my use is usually for reference anyway not really caring if there are one or two notes/ marks on a few pages.