Only three to go now - outstanding as some of these were quite difficult.
The last three clues now -
#250-5 - Three letters, the first two refer to the type of car and the last refers to it's make of engine. The car is Australian and the last letter is K.
#250-10 - This car is not very obscure, shares its name with a large lake in North America although it is European.
#250-16 - It's French and is based on a car beginning with the letter S.