Any Tomytec frame you get will have to likely be spliced longer, so remember that there are only a handful of variations on Tomytec despite lots and lots of different part numbers:
1) Trucks - originals have end gear towers, "R" versions have centered gear towers.
2) Truck wheelbase - look at the dimensions on on Plaza Japan, you're looking at 12mm and 14mm truck wheelbases
3) Pickup system - look at the trucks. The 14mm ones 'usually' have end-axle pickups, and the 12mm ones 'usually' have inside pickups (plates riding on the split axles). The end-axle pickups are more reliable. But the Doodlebug has a short-wheelbase truck, the 14mm ones are more like a 7' diesel truck.
4) Sideframes. Seems like all of them come with an assortment of sideframes to snap on, look closely at the package, but for the Doodlebug, I just cut off the original Bachmann ones and used those anyway. Easy to do.
The really early ones - TM01, KD25, etc. are shorties with only one-truck drive. Useful for my Whitcomb switcher, but even that will take the two-truck powered as you can always dummy down a powered truck to fit.
They've changed the motors as well, new ones are more of a smaller coreless motor and still work fine. If anything, the original motors (which were a dead ringer for some Bachmann ones) were subject to premature failure - only motors I've had that wore out the finger brushes.
Other than that, the chassis are all part numbered by length. Generally, for the doodlebug and the Metroliner, longer is better, but it still works. I'm just glad to see the shipping cost of these from Japan come back down to earth, it was pretty scary there for a while.