Buying a car as a secondary occupation, sole proprietorship

It will probably be light goods transport. That is to far fewer tax rules than usual. You can find more information here: https://financien.belgium.be/nl/private person/vervoer/inschrijving_en_taxes/lichte_vrachtauto#q2

You will have to record this, of course, as an investment and depreciate it over several years in your accounts. Probably 5 years. You can read more about investment in this article from Dexxter: https://dexxter.be/hoe-calculate-je-de-depreciation-een-cijfervoorbeeld/

How you calculate the payments is not really important for the accounts of your sole proprietor.