pick the brands you might want to use, ask for demo's so you can determine which one has the best features that you and your operators like best.
Then through word of mouth and some research find out what you can about the dealers service and parts. Most reputable dealers do have a rating and if you know anybody else that deals in management dealing with forklifts ask them how their servicing companies do.

Truck design and performance and dependability does play an important part in effective and efficient operation of your material handling but more importantly the quality of service you get from your representing dealership's service and parts dependability are a key factor on how that plays out as well.