I love when these types of threads come up.
People get quite opinionated and passionate about their work, which i don't see as a bad thing.
I personally use Plywood. I don't like MDF or melamine, that being said i've worked with companies that do use it. There is a difference in quality of the melamine finish and what it is adhered to, however i have no idea if there is in this country, i've never used it here and won't.
I have even worked with a product that is a mixture of plywood and chipboard core that was toted as very stable.
The thing to understand about melamine is that you need a scoring blade or you will be for ever routing edges to get it good enough as a finished product.
The next issue is edge banding, unless of course you are going with face frames. Iron on is not a good choice in my opinion after working at a place that had an edgebander.
In your situation i'd go with what others are saying on buying the carcasses, i've seen kitchens in £70-80 thousand range using melamine, don't underestimate the destructive powers of water on plywood, i've seen horror stories on both products.
As far as well built carcasses? God i don't know, i've only installed a few in this country Howdens, Jewsons, and *cough* B&Q and i wouldn't recommend any of them. So perhaps try the link someone else posted here.
You can resize them if need be so you can achieve that fitted look, and best of all you can splash on the doors and hardware.
I disagree with the comment someone made about kitchens being disposable. I personally know several people who have had theirs for more then thirty years and when one of them wanted a change they kept the carcasses and only got new doors. It's shameful that people waste so much, but as a bench joiner my income comes from such acts, so unless i stop taking jobs where people do this, i am just as much part of the problem.
If you want to build with melamine maybe you could get the sheet stock cut at a shop like Jonny's as he is set up and wouldn't take him very long to supply and cut the pieces you need.
There are so many options really. It's finding the one that fits. I've seen people being proud of some quite shoddy work they've done, so who knows at the end of the day. beauty is in the eye of the beholder.