* Don't be fooled by machine suppliers that offer machines at a low cost, but  actually do not have them in stock.
* Know your needs.
* Check their spare parts supply.
* Check their referral list, talk to their clients or better go & see a machine they sold in action.
* Check and compare their warranty and capabilities to stand behind the product they selling.

Check for any hidden charges like "structure options (own frame/concrete frame).