Hi,
You're really going to need to explore your options / needs / wants here.
Personally, I'd say go around to the various links in the projects section - most of them there have at least some of what you want, if not most. I'd say to take a look at Chris Ward's stuff, Lee's stuff, Adam's great 6502 stuff, and JDEBoy's (Joachim) information (JDEBoy's is through his posting to this board - post:
http://www.6502.org/forum/viewtopic.php?t=218 ), as well as information from the Speccybob2 design (the website seems to be down atm, but this guy has a wonderful little system that was based on the sinclair spectrum) All of these areas have tremendous information and are wonderful starting points. And if you have a question on how to make something work right or ideas on something, many here like Lee, JDEBoy, Garth, KC5TJA, and others will post and can help out tremendously! The people here are really great people.
Now for the not so great news.... Although most people here are more then happy to help out with ideas, concepts, "am I doing this right", etc. - I don't think most if not all here would be able to custom design a computer like you are talking about without something to compensate them for their time / parts / equipment, etc. - a custom designed system that isn't homebrewed for the love of it would be very expensive - especially if the market is 1 system. That's why most here experiment and work on their own designs and such - when doing it for ones self, it's a labor of love in the hobby, when doing it for someone else, most would probably need incentive to invest their time in something for someone else.
Now, this isn't to say you can't make an awesome system, or that people wouldn't be willing to help - I just think you're probably gonna have to come up with a lot of the design on your own tho. Plus, the huge benefit of doing it yourself - if something goes wrong - you can fix it. if something doesn't operate the way *you* want it to - you can change it to suit your needs and/or wants.
Take a look at some of the projects on the net - many have code to make them operate as well - almost all have schematics, which would make it possible for you to make one.
Just my humble opinion. Good luck in your design, and if you decide to piece together a variety of ideas from different systems, please bring your progress here or ideas/questions/suggestions, I'm sure help could be provided (I'm not necessarily saying I could help, I am not nearly as proficient or qualified as many of the other poster's here - I'd do what I can, but I would definitly defer to someone with much more knowledge then I to prevent causing more confusion