Ok, you have heard all of the others but i think you need some very simplistic clarification. When you are looking to replace your existing kitchen the best thing you can do is firstly measure your kitchen completely including were windows are from returning walls and the same with any doors and where the stop tap is and any gas mains, soil pipes, radiators, CCU's and serving hatches.
Once you have done this you can then start to design your kitchen by the number of base units you need remembering to leave space for appliances (dishwasher, wm, fridge etc) and aligning wall units to keep symetry. I would advise you to get an independant designer or a fitter who can also design to help you go over it.
Once you have got a fully planned kitchen you can than use the design to work out the number of base units, wall units, and work tops you need, this will be helpful should you wish to buy an ex display kitchen, however ex display are not necessarily the cheaper or better option so I suggest you seek sound unbiased advice. Any other q's I will be happy to advise.
Hope all this helps