Consider the following points by 42u.com while purchasing your server rack:
Cooling: Buy perforated server racks if the cooling environment at your location is ambient (fans, blowers, ACs). In case you are using liquid cooling units, its best to buy fully-sealed server racks.
Space: Select the server rack based on the amount of space you have. Keep unplanned future expansion in mind while you allocate space for the server rack.
Proper security: Buy server racks that do not demand too much change in your current psychical security settings. Server racks with locking front and back doors are a good option.
Customized design: Most server rack companies will have options of adding shelves, top and side panels, mounting rails and 'feet' to your server rack. Choose a design that suits you best.