Current list according to the wiki and personal experience as OP is missing a few of them:
Alfa Romeo P3 (Purchase Edinburgh Castle, automatically added to garage)
Barn Finds:
Aston Martin DB4 GT Zagato (Autumn Exclusive)
Audi Sport quattro
Bentley 4-1/2 Litre Supercharged (Barn Find Rumour: Purchase Bamburgh Castle)
Ford Escort RS Turbo
Ford GT40 Mk I (Spring Exclusive)
Jaguar E-type S1
Jaguar XJ220
Land Rover Range Rover '73 (Winter Exclusive)
Lotus Elise GT1
MG MGB GT
Mini Cooper S
Peel P50 (Summer Exclusive)
Subaru Impreza 22B STi
Triumph Spitfire
TVR Cerbera Speed 12
Car Pass:
BMW i8 Roadster: (Early Gameplay)
Honda Civic Coupe GRC (Early Gameplay)
Koenigsegg Agera RS
Mazda RX-7 Spirit R Type-A
Nissan Sentra Nismo
Porsche
#186 Prodrive Rallye 959
Porsche 356 A 1600 Super
Triumph TR3B
Events:
Jeep Trailcat - Cross Country Series level 4
Jeep Grand Cherokee Trackhawk - Cross Country Series level 8
Penhall The Cholla - Cross Country Series level 14
Ford RS200 - Dirt Racing Series level 4
Ram 2500 Power Wagon - Dirt Racing Series level 8
Renault Alpine A110 1600s - Dirt Racing Series level 16
Ford Mustang Boss 302 - Drift Club level 10
Chevrolet Monte Carlo Super Sport - LaRacer @ Horizon level 10
Lamborghini Huracan LP 610-4 - Road Racing Series level 4
Porsche Cayman GTS - Road Racing Series level 8
Jaguar XKR-S GT - Road Racing Series level 16
Noble M600 - Speed Trap Hero level 11
Ariel Atom 500 V8 - Speed Zone Hero level 11
Nissan Skyline GT-R V-Spec II - Street Scene level 4
Renault Clio Williams - Street Scene level 8
Hoonigan Rauh-Welt Begriff Porsche 911 Turbo - Street Scene level 17
BMW M3 - The Drift Run level 8
Dodge Challenger SRT Demon - The Stunt Driver level 10
Zenvo ST1 - World's Fastest level 10
Ultima Evolution Coupe 1020 - Drag Strip level 4
Formula Drift Car Pack:
Formula Drift
#98 BMW 325i
Formula Drift
#43 Dodge Viper SRT10
Formula Drift
#13 Ford Mustang
Formula Drift
#530 HSV Maloo Gen-F
Formula Drift
#118 Nissan 240SX
Formula Drift
#232 Nissan 240SX
Formula Drift
#777 Nissan 240SX
Best of Bond Car Pack:
James Bond Edition AMC Hornet X Hatchback
James Bond Edition Aston Martin DB5
James Bond Edition Aston Martin DB10
James Bond Edition Aston Martin DBS
James Bond Edition Aston Martin DBS
James Bond Edition Aston Martin V8
James Bond Edition BMW Z8
James Bond Edition Citroen 2CV6
James Bond Edition Jaguar C-X75
James Bond Edition Lotus Esprit S1
Forzathon Shop Exclusives:
Chevrolet Colorado ZR2 2017 (Winter)
Ferrari 812 Superfast (Autumn)
Porsche 911 GT3 RS 2019 (Summer)
Honda Civic Type R 2018 (Will appear in Spring Forzathon Shop October 19, 2018 to October 25, 2018)
Skill Trees:
Lamborghini Reventon Forza Edition: Found in Lamborghini Miura P400 skill tree
Maserati MC12 Forza Edition: Found in Maserati 300S skill tree
Mercedes-Benz SL 65 AMG Black Series Forza Edition: Found in Mercedes-Benz 300 SL Coupe skill tree
Nissan Nismo GT-R LM Forza Edition: Found in Nissan R390 skill tree
TVR Sagaris Forza Edition: Found in TVR Sagaris skill tree
VIP Membership:
Ford Transit SuperSportVan Forza Edition
Nissan Titan Warrior Concept Forza Edition
Pagani Zonda R Forza Edition
Wheelspin Reward:
2007 Alfa Romeo 8C Competizione Forza Edition
2017 Alfa Romeo Giulia Quadrifoglio Forza Edition
2015 Alumi Craft Class 10 Race Car Forza Edition
2016 Aston Martin Vulcan Forza Edition
1964 Austin FX4 Taxi
2010 Audi TT RS Coupe
2013 Bentley Continental GT Speed Forza Edition
2013 BMW M6 Coupe Forza Edition
2013 Cadillac XTS Limousine
1979 Chevrolet Camaro Z28
1969 Chevrolet Nova Super Sport 396 Forza Edition
2010 Ford Crown Victoria Police Interceptor
2011 Ford F-150 SVT Raptor
1993 Ford SVT Cobra R
1973 Ford XB Falcon GT
2004 Honda Civic Type-R
2005 Honda NSX-R
1972 Hoonigan Chevrolet "Napalm Nova"
2006 Hummer H1 Alpha
2014 Jeep Grand Cherokee SRT
2016 Lamborghini Aventador LP750-4 SV
1992 Lancia Delta HF Integrale Evo
1972 Land Rover Series III Forza Edition
2009 Lotus 2-Eleven
2005 Lotus Elise 111S
2013 Mercedes-Benz E 63 AMG
2012 Mini John Cooper Works GP
1995 Nissan Nismo GT-R LM
1973 Pontiac Firebird Trans Am SD-455
2018 Porsche Cayenne Turbo
2004 Subaru Impreza WRX STi
2014 Volkswagen Global RallyCross Beetle
2010 Volkswagen Golf R
1963 Volkswagen Type 2 De Luxe Forza Edition
1988 BMW M5 Forza Edition
2013 Caterham Superlight R500 Forza Edition
1972 Ford Falcon XA GT- **** Forza Edition (Censored by this site for some reason)
2012 Lamborghini Aventador LP700-4 Forza Edition
1980 Renault 5 Turbo Forza Edition
2010 Renault Clio R.S. Forza Edition
Peugeot 205 T16 Forza Edition
Not sure where found or simply not added yet:
Audi Sport quattro Forza Edition
Austin FX4 Taxi Forza Edition
Ford Capri RS3100 Forza Edition