The sentence seems to be missing some words.
Filling them in would modify the sentence into its more correct form which is given as follows:
He's buying A car TO go to work in.
A in the sentence above is an article. An article in English grammar refers to a determiner that may highlight the specificity of reference of a noun phrase.
To has been defined as a preposition that depicts movement. That is, it is best used to indicate when an object or a subject is or wants to change location from one point to another. To is best used with words such as drive, walk, fly, hike, sail, etc.
For more about articles and prepositions see the link below:
https://brainly.com/question/1649561
