The OP argued
Because when I say I am having a car, it could mean I am buying a car.
I'm sorry, no. The meanings of the two sentences are not interchangeable
In a restaurant
Server/waiter: Are you ready to order?
A: I'll have the fish
B: I'll get the steak
C: ? I'll buy the fish
In a bar/pub
C: What are you having?
D: Thanks, I'll have a gin and tonic if you don't mind.
When you want to pay for someone's drink, we ask: What are you having [to drink]?
You can also ask this at a restaurant when you want to know what a friend has decided to order, e.g.
Leo: What are you having?
Adam: I'll have the fish
The act of buying someone a drink or ordering a dish is a temporary one.
In front of a car showroom
Adam: I'm buying a car. ✅
Adam: I'm going to buy a car. ✅
Adam: I'm getting a car. ✅
A week later
Adam: I bought [myself] a car. ✅
After the purchase, Adam owns the car and can state:
Adam: I have [got] a new car. ✅
The act of possession is considered fixed even if cars can be resold and bought several times. But during the time a car is in a person's possession it is theirs permanently. It is highly unlikely that tomorrow, the day after and the day after that same person will buy a new car. And we do not normally use the Present Continuous tense to denote possession.
- ?I keep having a car. ❎
- ? I'm having a car. ❎
Sentence 2 suggest that the car is being eaten!?
- *? I keep on having a car ❎
Sentence 3 sounds like (I know it's impossible) the speaker is continually ordering or eating a car!
phrasal verb
1. informal: to continue happening, doing something, working, etc.
The rain kept on throughout the day.
He talked and talked. At one point I thought he would keep on all night.
She kept on talking after I told her to be quiet.
What the OP should say
If you still have that car next year, I'll buy it off you.
(an informal version of Kate Bunting's suggestion)
… a question mark in front of a word, phrase or sentence indicates that the form in question is strongly dispreferred [sic], "questionable" or "strange", but not outright ungrammatical.
Wikipedia