I use a 7518 in a Woodpecker's PRL...

1: Height adjustment from above the table without significant backlash
or horizontal spindle movement.

The PRL has an above-the-table adjustment that uses an
allen-wrench-head crank. I keep a dial indicator on mine when
adjusting, and I've never noticed any backlash or horizontal motion.
It's rock solid. It also has top-side height indicators for fraction
of a turn settings and zeroing.

2: Height adjustment that doesn't collect dust - either covered to
prevent entry or with a hole in the bottom to let dust out.

Both holes go all the way through, so no dust. I have a DC on the
cabinet too.

3: Spindle lock that does not require extending spindle all the way up
to actuate, but has a separate mechanism that can be activated at any
time. An alternative would be a two-wrench collet.

The PRL/7518 works by raising the router up enough to use the two
wrenches. Conveniently, the height crank acts as a stop for the lower
(spindle) wrench so you can focus on the upper (collet) one.

4: Robust lift mechanism that will last more than a year - prefereably
with no plastic parts to wear out.

All my parts are aluminum, mine's about four years old now. I
occasionally take the lift apart to clean out the dust and re-wax it,
but I do that to all my tools.

http://www.delorie.com/wood/projects/router/