Not so much a "guide" but a "Heads-up"
There are challenges coming up requiring specific cars so, if you don't own them already and are planning on buying them to do the challenges anyway, it might be worth getting buying them at auction for this challenge
(12th Jan - 19th Jan)
(19th Jan - 26th Jan)
(26th Jan - 2nd Feb)
(26th Jan - 2nd Feb)
(You win this car for doing the previous challenge so not necessary to buy)