The compressor for undercounter freezers can be located on either the sizes or in the rear. Its location will determine both the ideal placement, and amount of clearance needed. For side-mounted compressors, you will only have to worry about leaving proper space at the end the compressor is located on. Rear-mounted compressors will need either a few inches or back clearance or have vents in the front, depending on the model.
The overall width is important to look at to make sure the unit will actually fit into the available space. Be sure to leave factor in the necessary clearance on the sides when looking at a unit with a compressor on either the left or right side.
As an additional note, undercounter freezers should not be used as a food preparation surface. Though the tops may be made out of stainless steel, it is not food-grade stainless steel, so allowing raw food to contact the surface may lead to contamination.