Here is a non experts response:
Hard drive size is largely dependant upon what you intend to do.
Will the e-smith serve only as a router/gateway? File server for Windows clients? Web server? Mail server?
My experience? 400 MB hard drive is big enough to install e-smith 3.x for router/gateway and maybe a couple of web pages.
If several Windows computers will share files on e-smith, have a look at how quickly Win 98 users fill their hard drives! I would start with e-smith's recommended 3+ GB for that use. For a small LAN like mine, IDE is fine.
Ram? I haven't had a problem with 16 MB, but I haven't been using e-smith as a mail server. I believe I read a post from Charlie Brady advising some problems with qmail when operating with less than 24 MB ram. 32 MB is a good starting point on most newer machines because of the availability of 32 MB SDRAM.
Processor? 486 DX 66 MHZ runs well, but can be a pain to configure with the ISA motherboard. E-smith's automated installation works beautifully with PCI motherboards and NIC's from the supported list. You can search the forum for tips on installing e-smith on ISA hardware, but considering how cheap P-120's are today, it may not be worth it. Any Pentium should be powerful enough for a small LAN.
CDROM? Any IDE/ATAPI. Hook it up temporarily if you don't have a spare, and remove it after you are done installing e-smith. Or put an old 4X cdrom in it. You won't use it much.
You need a floppy drive, because it is required for the e-smith install. Nothing to use a mouse for.
After install, you can remove the monitor and keyboard for use elsewhere. Just about any old video card will do for the install.
But get a UPS. E-smith is Linux, Linux is Unix, and Unix doesn't like the power to go away.