How do i legally purchase a company already establish and what do i need to do for a proof of sale.

Is the business a separate entity, such as a corporation or llc?  If so, you can buy the entity.  For example, if it is a corporation, you would buy its stock.  Alternatively, you (or an entity you establish) can buy its assets.  For example, let's say that abc corp. is a taxi business owning two taxis, some equipment (spare tires, fanbelts, etc.), some goodwill associated with its name (which may or may not be a trademark), and has a lease on a garage.  You can either buy the stock of abc corp,, or you can buy all or some of the cars, equipment, name, and lease.  Each of these has different tax and liability consequences to both buyer and seller, each is accomplished slightly differently, and each may be appropriate in different circumstances.  For example, if the existing business has a lot of debt, you probably don't want to buy the stock, because the corporation would continue to be in debt even thougb you now owed it; in that case you would probably want to buy the assets, but you need to know what assets have been pledged to creditors as security for debt, and also the laws on successor liability.  You should probably consult a local accountant and/or an attorney.