Nothing is more important than knowing the exact measurement of the space in which you will be placing your new appliance.  Once you have these measurement, you will need to consider the following things:

• Build-in units - Designed to slide snugly in between kitchen cabinets.  These types of units need to have the exhaust vent in the front of the appliance where the air can escape easily. 
• Freestanding units - Designed to stand alone under the countertop. Compressor coils are on the back of the unit. Rear coils must maintain adequate air flow, so you'll need to keep several inches of free space behind and around the sides.  Refer to the Owner’s Manual for specific requirements of the appliance you are interested in. You may also want to know if the unit has adjustable feet.  This may come in handy if you want to raise the unit.
• ADA Compliant units - Designed to meet ADA height and government guidelines in public accommodations, commercial establishments, and assisted living facilities. These models are available for built-in and freestanding installation.  The units are typically 4” to 6” shorter than a standard 36” kitchen countertop.
• Door Clearance - The measurement that is required for the right or the left door with or without handles to be from adjacent cabinet walls so they can be opened 90°. Refer to the Owner’s Manual Installation Guide.

Location

To choose the right appliance, you must find one that meets the needs of that space.  Outdoor kitchen. Basement wine cellar. Indoor kitchen. Residential or commercial establishment. Institutional establishment.  Be sure to look for your specific location requirements when you’re reading through the features of an appliance.

Here’s a list of features that may be important to you:

General Features - 

• Installation - (built-in, free standing, outdoor, commercial)
• Appearance (stainless steel, panel-ready, black, white, etc.)
• Type of shelves - (metal, wood, flat, grated)
• Number of shelves
• Removable shelves 
• Reversible door
• Type of door - (glass-front, solid-front, panel-ready-front, drawer-style)
• Digital display
• Security lock & key
• LED lighting
• Energy efficient
• Noise level (dB)
• Location of exhaust venting
• Warranty

Then look at the appliance specific features:

Wine Cooler - 

• The number of temperature zones (single zone, dual zone, etc.)
• Temperature range - not all wines store at the same temperature
• Appliance’s ability to store wine long term
• Bottle capacity
• Bottle size requirements
• UV filtered glass door
• Compressor cooling

 Kegerators - 

• Number and style of kegs
• Number of faucets
• Essential components
• Conversion kits
• Cleaning kits and requirements 

Refrigerators & Refrigerator-Freezer Combination - 

• Capacity
• Use - (food & beverage, beverage only, produce only)
• Temperature range - Beverage refrigerators are typically colder than a standard fridge
• Ice cube maker - (with or without drain)
• Interior lighting

What do you like to drink?  Are you going to store food?

There are several different fridge sizes on the market, each one with its own storage capacity.  What do you like to drink?  Soda from a can?  Craft beer from a bottle?  What does your family like to drink? Do you collect or store wine? How large is your collection? Do you want to add to it over time? In short, consider your current collection and anticipate your needs for the future—from there, you will figure out the right fit.