sounds like the compressor is worn out SSW, beyond economical repair. Although its not a spring chicken at 8 years old, its still not that old...you need to check that the correct ventilation has been arranged in the housing.
Most require a large vent slot in the kick panel of the kitchen unit (can be covered with a decor grill) which allows air to travel under the appliance, up the rear of it (collecting the warmth of the appliance) and out through the top of the housing...bit like a chimney effect. Time and time again i see frs and fzs with no ventilation provided and they wheeze on for a few years before the compressor gets tired, by which time the installers are long gone.
All this will apply to your new machine too of course.
no need to worry about the top...if the kitchen units don't touch the ceiling then there is sufficient space behind the housing (built in to their design) and the warm air will just rise up behind and up towards the ceiling.
...as to replacing the compressor...well it is the main event in any cooling product, it requires major surgery to replace (gas tight connections need to be braized or crimped) and then a fresh charge of coolant gas installed. The cost of the compressor depends on its size but it will certainly not be a cheap repair!