First off, let me say I ran across this forum and site while looking to see what others were doing in the way of building a bench DMM, and I am bookmarking it as a new interesting source.
I end up doing a lot of traveling for work and have been wanting to put together a combined set of test instruments in a single bench package for when I travel. One of the key pieces will be a DMM since I hate having to leave my HP 3457A behind when I travel (it has actually come with me around the world at least once). Anyway, i figured I'd share where some of my thought processes on this type of project have gone.
1. Case- I agree with many on here that have discussed picking a form factor and building around it. I find this is key to not later having to try and stuff things in badly or ending up with huge cases. The simple act of defining a board or panel outline with constraints placed by the enclosure is to me very helpful. As to case manufacturers I have used most that have been listed before and I would add one more. For handhelds (although bench is definitely my preference I build a lot of hand held prototypes) I recommend Box Enclosures. My preference for them is they are one of the few that have rubber boots for most of their handhelds.
2. Design style - normally I try to make things as modular as possible, especially on prototype or low volume things. I actually was just looking at a spectrum analyser design that takes this to an interesting level
http://www.scottyspectrumanalyzer.com/slim.html is a page on their modular approach; essentially small modular boards with mounting solder pads on the edges that can be tacked to a large pcb with cutouts for a grid of the smaller pieces.
3. Included pieces - Personally my goals is to include a lot of different pieces of test equipment into on travel package, so not really applicable to the goals you have here, however I would suggest maybe adding one thing that for me is a big part of the design. I plan on adding a digitally controlled DC power supply since a supply is the second most used piece of test equipment on my bench after the DMM (maybe first). I was looking at doing something similar to
http://www.tuxgraphics.org/electronics/200707/bench-power-supply-unit.shtml but using whatever micro I settle on for the DMM and other functions. For my work the biggest upside to this is being able to see input voltage, current, and power all at once, and I will add logging since I build a lot of wireless battery powered items and need to see average power usage logged over various power states (sleep, transmitting, etc.)
4. MCU - For the DMM section I have narrowed my choices down to 3 possibilities. The first two are basic - ADUC845 or similar from analog (similar to the ADUC7060 mentioned before but 8052 based), one of the new PSoC's based on 8052 from cypress. Both of these offer precision ADC on chip with PGA's and MUX's as well as plenty of DIOs for controlling input, range and measurement type switching. The larger PSoC's also offer various filters and opamps on board, USB, and lots of flexibility for pinout and pin function (nice if you want to just break out a block of pins to cover various functions dependent on code). Partially driving these choices is that I have dev kits for both, and in the case of the aduc I have considered just integrating the dev kit board into the meter and saving a lot of design on the analog layouts. PSoC also has a new high precision analog dev kit that would make a good starting point for the meter.
My third possibility is something interesting if you are looking for very high precision. TI has a 32bit adc chip that they designed for very high precision measurements (ADS1282) while I personally would not even attempt to integrate this chip due to the LVDS signalling and need for a DSP to read it, they have a reasonably priced dev kit (
http://focus.ti.com/lit/ug/sbau144b/sbau144b.pdf) that includes a USB interface and sample program to do voltage measurements ($170 from mouser, not CHEAP, but considering the cost of 6+ digit bench meters a bargain compared to commercial meters). This board would be an interesting basis for a 6+ digit meter that has all the analog backend and computer interfacing done and just needs the front end range switching and whatnot developed. I am still strongly considering this as I do a lot of very low level measurements from sensors.
Anyway, I will be following this thread with interest as I think having more open source test equipment designs for hobby/small business electronics is a great idea. Computing power and precision measurement IC's are to a point now where there is not much test gear that can't be built on the cheap.
-Aaron