One of the main things to look for when looking to buy fitted kitchen units is the quality of the cabinets. The cabinets or carcasses hold and support the worktops and hold the kitchen together. One thing to look for is the back panel in the base units, are they a solid 18mm back. If so, your kitchen will last and look good for years. If the back panels are only made from hardboard with a thin strip of 18mm at the top after a few years of being fitted you could see some problems start to develop. The hardboard back could start to absorb moisture and start to bow out inside the unit, this can make the unit weaker which will show on the doors starting to drop and look out of square. Also make sure the edges of the cabinets are full sealed and edged, this again will help stop moisture seeping in over time and damaging the units. Do the cabinets come with quality fittings? Blum hinges are one of the best hinges coming with a life time warranty.