What is collectable?
Generally any R/C kit from Tamiya is going to be a collectable eventually. Tamiya tends to rollover kits on a fairly regular basis. With the exception of some models which seem to linger forever (Clodbuster, Midnight Pumpkin and Blackfoot), most have a short lifespan.
But before I delve any further, let me give you some advice. If you're in it for money: get out, because there's not much to be made here. You're better off placing your money in mutual funds. Always collect what you enjoy because the market does fluctuate and what is worth a lot of money now may be worth very little in the future.
Factors that determine collectability
Age - Obviously this is going to be the key determiner. The older the kit generally the more its worth. This is especially true of mint unbuilt kits. Their price tends to rise much more quickly than their built counterparts. But age can never be used as the only factor. You must always factor in the other aspects.
Condition - even cars that are not that collectable have an appeal if they are mint or new unbuilt. Because these were not sold as static models, it means that a mint kit is still a rare bird despite high volume production runs. And there isn't a collector out there that doesn't enjoy the process of building a car up from a brand new kit!
Rarity - If the kit was generally in short supply or limited run, they tend to be more collectable. A good example is the Tamiya Avante. A dismal sales flop when it was first released, now mint in box editions of this car fetch more than twice what the retail was when it was first introduced. Over time the limited availability of these kits will make them more collectable than their regular counterparts. Having said this, I must point out that this is not always true. The car must have at least enough of a production run such that there is public "awareness" of its existence. If the kit was so rare that no one remembers it, then its collectability is a little lower. A good example is the Tamiya Solar car. A full R/C solar powered runner, it was never imported into North America. With a retail price of $500 U.S. in Japan it was also one of Tamiya's most expensive kits. Yet I remember a mint built car coming up for sale recently which didn't sell even at a 1/3 of what the original price was.
Desirability - This is something that is rather difficult to measure. What I am referring to here is how popular the car is amongst collectors in comparison to the number of cars available. For various reasons, some R/C models strike a chord with collectors and enthusiasts, these cars fetch more on the open market than what their availability would indicate. A good case in point is the Tamiya Bruiser. The Bruiser was not a low volume truck, it sold very well for Tamiya and had a long production run. But on the used market, a Bruiser will always sell well, no matter what condition it is in.