I think that you would be better off buying the 2J. I do like the Toyota 7 however you can't really use it for anything else. The 2J is a cool car and can be used for everything especially seasonal events. It's amazing. Just do the RB standard championship twice and you can buy it!
I bought the Toyota 7 specifically for the Historic Racing Cup race at Monaco. I grabbed the win, but quickly found out that the Toyota 7 isn't a very well rounded car. I don't think I've ever used it since that racing series. If you have the credits, the 2J is an attractive option. It's one of the infamous "ringer" cars in GT6, but it's not cheap.
I suggest that you buy both the Toyota 7 and the Chaparral 2J - they are both great in their own way and are the only Group 7 sports cars in the game. The Toyota 7 is harder to drive and laps slower than the 2J so if you are short of funds get the 2J instead.
I enjoyed the Toyota 7 so much that I ran a championship for it, but it is difficult to drive - it wheelspins on race hard tyres in the lower gears and possibly because of the short wheelbase it wants to swap ends on the brakes - you may find you have to brake in a straight line and drive it a bit like a giant kart. In real life it must have been difficult to drive - the project was cancelled after accidents which killed two test drivers.
Another great car to consider for the Historic Racing cup is the Chaparral 2D.
Regarding the Dodge Challenger race car (1970) - the Chevrolet Camaro race car (1969) laps faster than the Dodge so pick that if you are short on credits.