So... after many... well, I have to admit years of wanting one, I am going to actually start making my own Astromech.
Firstly, the legal: The design is owned (as far as I know) by Lucasfilm... or whoever. It's not mine, is the main thing. The look of the Star Wars astromechs is copyright.
Secondly: Don't ask me to make one for you. For starters, I don't have the time. Additionally, you don't have the money that you would have to pay. Also, the fact that the design of them is patented means that the money would go to George Lucas, via the Clone Army of lawyers that would be launched. However - Mr Lucas has gracefully allowed fans (such as myself) to unleashed our creative and/or technical talents in recreating elements of the SW universe.
Now that we have that out of the way...
What strikes me is that the internal, electronic systems are an interesting (albeit potentially major) engineering project. The stuff beneath the shell is up for grabs - think of it as building a robot, except having an external design to adhere to. External sensors are limited (for the most part, I might have to fudge) to what can be seen, but under the skin has plenty of options open.
So - Why an astromech?
Simple. I'm a fan of the rotund 'bot. Yes, I know how much "personality" was not much more than puppetry, but they are an interesting design. Yes, using a set design makes it more difficult... but I'm claiming that as part of the challenge.