I guess "must have" is relative but this is how I see it.
Absolute must haves:
*Alfa Romeo Gr.4 car- best Gr.4 car in the game
*Gr.2 McLaren- costly but best legendary Gr.2 car I think
*One of the three Group C cars- Sauber, 787B, Jaguar (I think Sauber is the best but there are varied opinions on all three of these, nonetheless, they are ALL good)
Great to have:
*The other 2 of the three best group C cars that you didn't buy above. They are all good cars, get them all if you can
*CLK-LM- great car but I think McLaren is just better
Get if you have money:
*Gr.3 McLaren- it's good, but not the best, and it's expensive
*McLaren F1 (road car)- nowhere to drive it but c'mon, it's probably best car ever made
*Tom's Supra, Pennzoil GTR- classic Gran Turismo and they are relatively cheap but they aren't any better than most Gr.3 cars so you're really only buying for the nostalgia
And I got one wild card here- Mercedes S Barker! That car is so fun to drive! I know it's really expensive, but it's so amazing! This might be the most fun I've had in GT ever driving this thing. It does not drive like an almost 100yr old car with a luggage trunk on the back. It handles like a dream, competitive with modern cars. It's amazing that you can beat modern cars with a suitcase strapped to the back of your car. Seriously, you will not be disappointed.