With a large briefcase, on could have a portable workbench that he could carry around and work on projects as time and place permits. It might be a little more costly in some cases because of the miniaturization that might be needed, but not always. The heart of the workcase (briefbench?) would be the DMM. If you are primarily interested in computer/controller devices, the cheapy mentioned before is fine. A Proto board will be useful in most cases too. With that, you need some power so either a battery pack or a small power supply and long cord will be needed. I suggest 5v and 12v as a minimum and –12 as good addition. One amp for 5v and ½ for the two 12s. Get some of the little parts boxes with lids for all the little screws, transistors and other parts that one needs. Attach with Velcro. Make the briefcase so just opening it allows you to work (if possible). A removable tray with tools or projects is fast and easy if you expect to have some room to spread out.
Instead of the cheap DMM you might consider the pencil DMM as it is smaller, or the RadioShak credit card style DMM. For more demanding work, consider the LCD oscilloscopes on the market. I got a pencil o-scope for about $100 that is great for low frequency (audio), low voltage (100 volts) projects. Do your homework on these as there are so many to chose from and many are older technology that cost too much for what they deliver.
Another option is the old laptop computer. Old makes it cheap and also makes it more likely to have a serial port. Serial ports allow you to hook up the computer to many projects/devices and also to allow the laptop to be a lab instrument by hooking up an adapter that gives the computer the power of many lab instruments in one at a significant savings. And if you are doing digital or controller design, the computer provides the programming interface needed too. If you go this route, I strongly recommend doing only battery powered work. It is far too easy to get into ground loops and hot chassis and fry everything. If the battery wont last long enough, get 2 and charge one as you use the other. Try to get very efficient battery converters and power supplies as batteries seem to go flat just at critical moment. I will be trying to bring you some power supply projects in this series of articles.