I was able to find a pretty detailed guide here, but just in case that link goes down:
EDIT: Note - these barn finds may not be in order as I found #9 first, as did Rapitor
Barn Find #1 – Lamborghini Miura P400
Location: To find this car, you need to travel to the Horizon Festival and travel to the south side of the ‘HF’ pink blimp on your mini-map. You need to travel along the dirt road with wooden barricades on the sides.
Eventually, you will come across two buildings and a garage door in
the middle of these buildings. Stand in front of you to find the car.
Barn Find #2 – Ferrari 250 GT Berlinetta Lusso Location: This is a
pretty hard one to find, but if I can manage it, anyone can. To get
this car, you need to travel to the north side of Casteletto and
travel to the smaller circle showed in the image. You will find the
barn right next to some buildings near trees.
Barn Find #3 – Ford Capri RS3100
Location: You will find this car in the southeast side of Montellino. You will find a tractor parked inside a shed near the location of this garage. In addition to this, the area also contains an XP board.
Barn Find #4 – Volkswagen Type 2 De Lux
Location: This is another tough one to find. To find this, you need to travel to the north side of Montellino and stick to the west side of the dirt road near the area provided in the image. The garage will be located near some buildings enclosed by small stone walls.
Barn Find #5 – Jaguar XK 120 SE
Location: You need to travel to the upper side of the San Giovanni Car Meet icon on your mini-map to find this amazing car. Travel up the hill and get to the right side of the ‘P’ icon to find this rusty old car.
Barn Find #6 – Ferrari GTO
Location: In order to find this car, you need to travel to the small circle in the image and turn right from the intersection. You will come across a large dirt road on your left side with another small/hidden dirt road a little ahead. You need to travel to the smaller dirt road on the left side and keep on following the path to find this car.
Barn Find #7 – Jeep WILLYS MB
Location: You need to travel to the designated location and you will come across a road with a small white stone border on one side. You need to find a gap in this border and go a little right from the entrance to find this car.
Barn Find #8 – Alfa Romeo Giulia TZ2
Location: Get to the area showed in the image and turn right from the intersection. Keep on going straight and you will come across large blue truck parked outside the barn find.
Barn Find #9 – Renault Alpine A1 10 1600S
Location: To get this car, you need to cross the river and get to purple fields. Do note that you need to stick to the purple side of the dirt road to find this barn.
Barn Find #10 – Maserati Tipo 61 Birdcage
Location: You need to get to the location shown in the image and go through the woods to find the lone barn up the hills.
All credit to Arslan Tufail at the site listed above
The flexibility of the car classes is designed so that you can race a wide variety of cars in whatever class you enjoy racing in. However, you shouldn't necessarily upgrade every car all the way up. Just because you can upgrade some "D" class car doesn't mean that it will be competitive as an "A" class. To really determine what class you want your car at, you have to examine the other cars you will be racing against and try to beat their stats not just their class. Also, it's very interesting (to me, at least) to race in lower tiers because the cars tend to be very closely matched so precise driving becomes important.
Just to note, the reason that cars don't perform well in every class is because each part contributes to the overall class for the car, but they don't necessarily contribute the same boost to performance.
All that said, if your difficulty is not set that high and you want to go fast, then go crazy with your upgrades and you should be competitive for the most part. If you're racing at the level where you must match yourself against the other cars then tuning set ups will be more important than the class that you upgrade to.
Best Answer
Unlike Forza Motorsport 5, You still need to purchase the cars in the carpack with CR (First time is usually free). The Day 1 carpack for example are all cars already purchasable as you play through (for the same price as their boring counterparts), but they have a unique livery on them. Purchased Carpacks may be different. I haven't spent enough time in the carshow to be sure ;)
EDIT: It seems there was server issues at launch and car rewards may not have been given out.