It is better to provide over rather than under-capacity of cooling because it imposes less strain on the compressor. And insist on high-quality equipment, especially the controlling thermostat. A faulty thermostat could cause the compressor to operate non-stop, leading to temperatures of 10 degrees centigrade or more below zero, with absolutely disastrous and irretrievable consequences on your wine. Provision of an alarm set to go off if the temperature goes beyond pre-set minimum and maximum temperatures is essential. Avoid fluorescent illumination which ruins wine.
Adequate lighting helps to locate bottles.
Some form of racking and shelving is needed. The choice is very personal, and depends on whether it is a show-case type or a working cellar. There are designer-built racks of hard wood which will provide an impressive display but are very expensive. This would provide storage capacity for 3,000 to 5,000 bottles, depending on the type and style of racks and shelving. Always get more than one quotation, and, final advice, use only engineering-qualified contractors.