Also, (4) not running a full desktop may make sense the old target machine doesn't have much RAM ( more RAM will be available as e.g. with a menu (console-based or graphical) to a list of (2) highly relevant tools that wouldn't be included in a generic live system, or (3) tools to fix problems on other widespread operating systems. Being dedicated to a smaller subset of tasks, they can (1) often guide the user, e.g.

not savvy enough, not sure about the work to be done, cannot access the Internet), it might make sense to use a dedicated rescue live distribution. I actually happen to often keep in my work bag some USB sticks with Xubuntu and they saved the day sometimes. assuming the machine can access the Internet. This has the benefit that once booted on the machine, even if a needed tool is not on the ISO, one can use apt-get or aptitude to get tools from the Internet. If for your rescue task you can figure out the tools you need, find them use them on your own, then a live Ubuntu (or derivative system) is handy (or any environment you're already familiar with for that matter). That said, since you mention rescuing an old server: Other solutions considering your need: 'old server that crashed' So, if 700MB is a hard limit for you, use an ISO of the first release. You can see that nearly all desktop ISOs of the first release (i386, amd64+mac, amd64) are smaller than 700MB (683MB to 694MB). This link: Lubuntu 14.04.3 LTS (Trusty Tahr) shows the current Lubuntu ISO sorted by size. I personally booted Ubuntu, Xubuntu, Lubuntu from live USB (and in older years, CD) to get a graphical desktop and run some tools, even sometimes on machine without storage.

The regular (not the alternate or server) ISOs of major flavors of Ubuntu, whether 32-bit or 64-bit, can all boot live on a machine to test drive without installing. 'boot without changing anything on the HD' Since you ask on AskUbuntu, here's an answer to what you're asking for.

Download ubuntu 14.04 live cd