What radar technology and who makes an appropriate set of radar
components needed to build a short-range radar system with the
following specs:
Min Range: <50m
Max Range: 3000m
Best case area of detection (possibly requires multiple sets of components)
360 degrees around
10-30 degrees elevation
Capable of determining distance/direction (and thus deriving speed
from multiple samples) of a
6-12inch diameter target 
moving at a rate of between 100m to 600m per second
with minimal lag (<100ms, the smaller the better)
with output suitable for digital processing (rather than user-display)
and minimized power consumption for above specs.

I've found some interesting stuff about UWB radar from IEEE and
related sites, but can not find part makers and I'm not sure it would
have sufficient range and/or area of coverage.

I'm trying to figure out the feasibility of building a relatively
low-cost, off-the-shelf components force-protection device capable of
intercepting RPGs in-flight, as a personal project.

Hello,
Building a low-cost radar is something that many people want to achieve
but fail. Usually low cost means poor quality.

I cannot answer your question but here is what you can do:
You can contact companies / labs that make radar systems
over the phone/email and pretend that you want them to design 
such radar for you. When they tell you the cost and parts involved, 
you may judge how easy or hard it would be to do on your own.