Little Project - a 2-Level "Porch Light"

My front porch light, motion activated, developed
a 'fault' long ago - one I *liked*.
The motion sensor did NOT cut off entirely.
This left the bulb running at like 25%
power all the time. IF motion, then it'd
go to 100%
This was actually GOOD.
Alas this fixture is OLD - and has now
trended towards "blinking" if the temp
goes below 50F.
Bought a new 2-bulb fixture. It's "dumb"
but I want to smarten it up.
My PLAN is to add a motion sensor, but
to ONLY one bulb. A rather low-watt
yellowish "bug bulb" will run ALL the
time. A bright 18-20 watt LED bulb will
be attached to the (added) motion
sensor.
Doesn't seem TOO hard.
I'm not adverse to weird wiring or
drilling holes in things.
BUT - any advice/notes ?
The choice of the add-on motion sensor
especially. Need nearly a 180-degree
sensitivity and a great service-life
profile.
Um, sorry, this doesn't need Linux or
micro-controllers or anything - just
real-world simple stuff. LONG life
is the goal.
--
033-33