Fist of all, I have to give up idea of universal DMM. Designing DMM is not trivial task, especially when it comes to mechanical design, good reliable switches, overvoltage protection and other stuff that makes DMM reilable nad safe. There is a lot of cheap DMMs with poor protection and those are dangerous in users hands. I'm not going  to try to beat years of experience of Fluke or similar brands.

What I'm going to do is to design simple and affordable device with measurement ranges that do have sense for Progmeter. My aim is to measure voltage up to cca 100V - I believe most of the hobbysits dosen't need more for 99,9% of their time. For higher voltages you can always use your normal DMM (as you probably won't need the Progmeter functions), as Progmeter isn't going to replace your DMM. For measuring resistances, my aim to is measure 10R to 10M with accuracy of at least 0,5% or better (it will be tough for two wire method, especially at lower resistances). I'm not decided about current measurement for now; it could be handy, but on the other hand I have to admit I use current measurements only for checking whether my device sucking a lot of juice or not. I don't feel like this is somehow important parameter for progmeter function - and you can always emulate it with measurment of voltage drop on resistor. Progmater is especially suited for calculation of current from voltage drop.

At first I wanted to use some nice OLED display, but I decided for LCDs due to lower power consumption. Something like eadogm132 looks nice. It can display four lines of text, as well as HUGE characters needed for DMM function. Tiny characters are nice, but when working in worse light conditions or in the need of checking the vlue on display from distance of few meters are big character definitely better.

I'm going to replace the readily available keyboard for matrix of normal SMD switches, something like this. It is very hard to find some keyboard with more than 16 keys.

ADC used will be likely MCP3550, it is cheap and simple to use sigma-detla ADC, with truckload of precision for this application.

For MCU, I'm going multiplatform again and write firmware and design PCB for some PIC micro as well as AVR8 or some tiny ARM.

Now I can start drawing schematics and place components on PCB.