I don't think you can use 'I am having a car' in the sense of owning it. No English speaker would say that, the same as no English speaker would say 'I am knowing that'.
Do you you have a car? *Yes I am having a car. Yes I have a car.
People from India do tend to speak like that when they speak English. I think it must be something they are translating. Sounds sweet!
I agree with corum 'having a car' is a compound noun, and is the object of the preposition 'in' 'in having a car' is an adverbial pointed at 'interested'
You can't even use owning in your sentence: I'm owning a car.
This is just a guess, an attempt at an explanation. The adjective 'own' comes from the OE 'agen', related to German 'eigen'. The verb was 'agan' 'have or own' and a long way down the language road, to the PIE '*aik-' with the meaning 'to be master of, possess'. If you could be said '*to be owning a car' you would still be 'mastering' it. Thus, you would not actually be master of it yet, and there is a semantic contradiction. To avoid this contradiction, English never developed 'am owning a ...' A similar argument could be put forward for 'am having a car' in its sense of possesion.
Just my idea on this, not to be taken as the truth, the whole truth and whatever. Maybe some Hindi or other Indian language speakers could tie this in to their language.