I use a factory-standard Sun SPARCstation 5/85 with 64Mb of RAM and, as mentioned, a 535Mb hard drive for Linux. Installing RHL on this was a breeze, although I have since gone back and customised it heavily, removing unwanted packages and adding others such as the C++ development environment (for package source compilations).
SS5s are still supported by Sun but no longer sold. You can probably pick them up secondhand now for a reasonable sum.
I also chose AfterStep as my desktop environment and window manager. This was mainly an arbitrary decision based on what was immediately available and up to date in Red Hat. However, I certainly didn't want to use FVWM95, KDE or anything reminiscent of the Win95 interface (which is a dumb choice of model for a UI - the main justification I hear for this is to make the Win95 weenies feel at home; arse). GNOME seemed to be too immature for serious consideration, and there were a large amount of supporting bits required (I hate packages that have a long list of prerequisite installations). Enlightenment looks gorgeous, but who wants a pretty face when there's work to be done? I realise that I could have adopted OLVWM again, but that would have meant tracking down OpenLook libraries and recompiling source, tedious tasks that I wanted to avoid for reasons of time. (As it turned out, this problem would bite me again and again under Linux.) I also sought to make the overall experience more novel and emphasise that I was using a different system.
Arguably, by choosing AfterStep, I made life harder by forcing myself to adapt to an interface that was radically differed from the one I previously used. Indeed, a large amount of time was expended tweaking AfterStep to behave as I desired. I suppose this has served to keep me on my toes and test my adaptability.
Everything else I wanted was available in Linux. I use the most recent release of Mutt, which is not totally backwards compatible with earlier versions, but since I run that on the (Solaris) mail server anyway, I didn't need to recompile it.