For cpu I'd go with a quad core i5 cpu, one or two price notches down from the top of the i5 range.
I'd go with 8GB ram, 2x4gb sticks.
Onboard graphics should be enough unless running CAD software or needing to drive an unusually large monitor or more than 2 monitors.
Windows 7 home premium or pro 64 bit OEM will be very usable unless the person using the computer needs to be in the windows 8 environment for some reason.
If space is an issue, get a micro ATX or even a mini itx size mobo and case. Lots of reasonably attractive small cases out there for under $50.
Do NOT go cheap on the PSU. A 450w seasonic modular psu would be a good investment. Save a few bucks by not going modular, at the expense of having to do some extra work during the installation to wrap up extra power cables and tuck them out of the way so they don't kill airflow in the case or rub up against fans.
When I built my wife's work computer, I put in a $25 DVD burner and a $20 20-in-1 flash memory reader in one of the 2.5" drive bays. I didn't use a floppy drive. Some of the 20 in 1 flash memory devices also have a floppy drive built into them, if the user needs a floppy drive. Otherwise USB floppy drives are fairly cheap.
The stock intel heatsink is "good enough".
If network throughput will be important, get a mobo with an intel networking chip.