Here are some comparison photos:
Type 87 (~1942):


Photo on left is a Type 877; it has the wrong rims. You'll notice the Type 87 has no front bumper - there is a roller in front for helping climb over brush. The horn vent next to the left headlight is also larger than in later models, and is not a chrome vent. Early Beetles also used semaphores located in the door pillars instead of turn signal lights as you see on modern cars (or on the fenders of the 1971). Early Beetles also had only one exhaust pipe, not two as you see on the 1951 and the 1971. Notice also the bumpers of the '51 compared to the Type 877. The 877 has a flatter ribbed bumper (ultra expensive to obtain an OEM set) as opposed to the rounder non-ribbed later models. You can see many differences between these two and the 1971.
In fact, between 1950 and 1970 there were already over 1000 changes that had been made to the Beetle's design, but they were done in such a way as to make most every part fit any year Beetle (this idea was crushed when VW began exporting to the US and had to begin dealing with safety and emissions regulations here). That's why they all look alike to most people.
1951 Sedan:

1971 Sedan:
I'm sure with a little work you can get the 1/18 to pass muster for your purposes but it'll take a bit of work to get by a VW nut and thankfully there aren't any of those on this forum

. For a real challenge, you could try converting a Beetle to a Kubelwagen. The floorpans are very similar but things go awry after that point.