Do you really want to go back to the situation of several years ago: Very few manufacturers and one dominant one, little innovation and changes in the design. Sure I bet racing was fun back then, but design-wise it was a pretty boring class.
A standard class is nice for close racing, but consider this: When the stakes are high enough, racers or manufacturers will find ways to bypass regulations to go quicker, so to keep the playing field level, either everyone must implement these bypasses, or they should be forbidden. Before you know it, racers will implement approaches to going quicker that are very hard to prohibit as they are difficult to control - a nice example is the whole tire situation around the last EFRA Euros, or the amount of trouble people go to to have a dominant pace in stock classes
Is that really what you want the 2WD Buggy class to go to?
Looking back to the whole goal of standardisation, it is to make the field more closely matched. Of course, regulations that are too open will mean that a big budget will allow you to buy a car for every surface, but you can't equal out differences with regulations entirely... so don't, it will only add grief to a sport that's intended for the highest fun-factor.
Instead, I think Janus' comment to restrict the amount of different cars someone is allowed to run at one event (or even during a racing season) is quite a clever one
Also, here's an idea from me: Make seperate series for high-bite and low-bite tracks, or clay/dirt and artificial surfaces - that also prevents the confusion of what sort of car to run, especially in combination with restrictions in switching cars during an event.