下列哪種說法比較自然？它們的意思又有分別嗎？

(1) How did you buy the car?
==> 你是怎麼購買此車?
That means, you have already bought the car. I ask the question as I want to know the way 方式 that you used to buy the car.
The answer could be:
- I bought the car directly from the wholesaler.
- I bought the car under the hire and purchase scheme.

(2) How did you come to buy the car?
==> 你怎麼會(想)買這輛車?
come to do something : used in questions to talk about how or why something happened (Oxford dictionary)
"come" 與 "how" 連用, 構成問句 = (怎麼)會的...? (Yahoo dictionary)
You may or may not have already bought the car. I ask the question as I want to know the factor and the reason 因素及原因 that led you to the thought of buying the car.
The answer could be:
(1) I've decided to buy it because it has been my dream car.
(2) I bought it as I thought the price was good.

哪種說法比較自然？
I think it is more natural and common to ask the second question than the first one, as it is more specific (明確/具體) as a question.

看來不是自然不自然的問題
兩句的意思完全不同啊！

How did you buy the car?
你是怎樣把(這)車買回來？
你如何買下(這)車？
[中文不作多譯過去式了]
意思是：你用甚麼方法(通常指付款方式)買這東西。

How did you come to buy the car?
如果照字面解：
你怎樣來到(這裏)買這車？
意思是：你來買車是坐車/走路來/有人載你來...？
另一個含意解是：
你為何想買這車？
但句子卻寫錯了文法：
How did you come to buying the car?才是這意思正確的寫法。否則句子是上面字面句子解。
其實come to是省略了的寫法，傳統或全個phase 可以有多種附加指示的意思：
How did you come to think of buying the car?
How did you come to the idea of buying the car?
How did you come to the decision of buying the car?
etc.

How did you buy the car=As it happened,,how come you buy a car?
How did you come to buy---=come to a decision to buy it to travel.