If you don't mind having the choice between Linux or Android there is a brand new system available based on a Freescale ARM by boundry devices. This may take some technical expertise to get some things running on it. If you decide to use Linux you may have to recompile things you want to use for it. If you are building it yourself you need a case such as the ByByte case available online here.
If you purchase one of these:
And add either of these screens:
$225 Capacitive multitouch
You can then add one of these cameras:
(USB web cams have also been known to work as well. Just have to do your homework.)
For storage they come with 2 microSD slots. One will have your OS on it and the other will be available for storage.
If you really need storage then you want one of these connectors so you can add a SATA drive.
Or you could add a PCIe board with this add on:
(PCIe can do MANY things.. This is a laptop/netbook internal expansion port that most laptops now use for wireless network and bluetooth)
If you want to add buttons to it for use with android or something you can get one of these:
The cool thing with this is it is a quad core ARM cortex 9 running 1ghz with 1gig of ram. If you add any of the devices mentioned above they will work great with the Linux or Android versions available. This unit is quite fast and you could do much more with it. But you can't be afraid of recompiling code to work on this since it is a more specialized device than using a well known tablet. This setup is what I am building for both my truck and my car although I will be mounting the screen in the dash and mounting the rest somewhere else.