Can I purchase an existing Thai business while on a DTV visa?

Why not...

So i can buy 1-2 condos and rent them out ?

Yes, you can own shares in a Thai company while on a DTV.

But DTV does not give you the right to work in Thailand. If you purchase an existing Thai business and want to actively manage or work in it, you would need the proper visa and work permit (usually a Non-B + work permit).

Also remember Thai company ownership rules (foreign shareholding limits) still apply.

Yes.

Yes, you can purchase shares in an existing Thai company while holding a DTV visa.

However, DTV does not allow you to work or actively manage the business in Thailand. You may act as a shareholder (passive investor), but if you plan to operate or manage the company, you will need a Non-Immigrant B visa and a Work Permit.

Also make sure the shareholding structure complies with the Foreign Business Act.