Hi - The '99 MS has 143k on it, and as expected - doesn't feel great in the front end.

Most noticably there's more play in the steering wheel than I feel there should be. I can reach through the driver's side wheel well and move the shaft back and forth a few degrees (5-10?) and see absolutely no movement on the output side of the steering gear (my '95 with ~200k had a similar problem).
Also I'm noticing some knocks/pops when it hits bumps at low speed.

Would it be worthwhile to do an entire front end kit (all 4 ball joints, all 4 tie rods ends)? What about pitman/idler arms?
Is there a particular part I should be focusing on - considering the symptoms?

I assume if I'm going to tear into it the wise thing to do is just replace everything - especially given the mileage.

General thoughs or suggestions, tips for completing the work?
How much time should I give myself - being a semi-competent DIY'er, but without much experience doing front end work (an inner cv boot on my '95 LS)?

You can take up some of the steering box slack with the adjustment screw on top of the steering box. put the steering in straight ahead, loosen the lock nut on the adjusting screw, and tighten until the slack in the steering wheel shaft is almost all gone. If you get it too tight, it won't self-center on the road coming out of a corner, and you'll have to loosen it a hair.

Next most likely slack spot is the idler arm shaft bushings, which are quite easy to replace.

Next is the pitmann arm ball joint.

My front end guy puts a floor jack under the lower a arm and lifts just enough to unload the tire, then uses a long pry bar under the tire to lightly move the suspension joints. Also possible to lie underneath and look closely at each joint as somebody flexes the steering side to side about 1/8 turn each way. A worn idler arm shaft bushing will show as a rocking of the shaft. Other joints show slop in the 1mm range when moved. Another check is to compress the tie rod ends, pitmann end, ball joints, and idler arm end with a big pair of arc joint pliers, and measure the stud height compressed. Take off the pliers and measure the uncompressed height. A diff of 1mm or more is worn out.

Also check wheel bearing slop at that mileage by wiggling the wheel on the spindle, grabbing at 3-9 o'clock. No slop should be felt.

Ball joints on these seem to last a long time, as do a arm bushings, if you lube it right.

To check wheel bearing play, shouldnt you push and pull the 12 and 6 o'clock position with the wheel off the ground? On mine, the pitman and idler arm have some play, it would be hard to tell if the wheel bearing had slop.

I was taught to use 3/9 to avoid misdiagnosing worn ball joints as wheel slop, and to use a light touch at 3/9 and watch the tie rod while you do it to sense wheel movement without steering movement.

Here's an on the road diagnostic for idler arm play. Worn idler bushings put slop in the pside wheel steering, and NOT in the dside. The pside tire will toe slightly out while driving forward. As you start a mild turn to the left, that tire will resist the turn, then flop into the turn, causing a mild dart to the left. Truck will pull slightly to the right when going straight. Steering slack feels looser in right turn direction that in left turn direction. Slack is abrupt to the left, and has increasing resistance as slack to the right is taken up, if measured while moving in a straight line.