What I read elsewhere in short indicates the following:
#1 You want to grab several attachements immediately, so PUP has a high startup cost.
1 - I agree with spiritreaverdiabolos. You should give it a try with the NPC vendor items (plus a Flashbulb from the AH, probably about 10-20k), which provide a nice mix of what PUP can do. If you find that it's something you want to progress on, you can add more. Once you start partying you'll want to grab commonly used DD-focused items (generally fire and thunder based), and you can put together some very solid DD attachments for not too much gil (say, Target Marker, Attuner, Stabilizers, Tension Springs). Coiler is a little more pricey (150k-ish on my server) but very useful.
The items that drop from the Targeting the Captain quest are the expensive ones, and some of them are really good. However, those are sort of the signs of a dedicated PUP. I'd expect someone at endgame who cares about their gear to try to get them, but by no means would I expect a mid-level player to have them. And to give a little less fear, some of the super-expensive items really don't even become highly useful until more "endgame" situations (Spiritreaver nuking = Condenser and Tranquilizer help a ton). Turbo Charger is the most pricey attachment that's truly useful from an early level, but if you can't get it you can still be plenty capable. My general rule of thumb is that once you get past AF levels you should probably be trying to fill in any gaps in your attachments.
#2 The Ranged Automaton seems to be the clear choice pretty much until end-game? Where it's replaced by the BLM?
Kinda. The ranger frame (Sharpshot) is the primary exp/merit party frame. It's pure DD, gets really strong WS (once you get Daze in the 50s, look out!), and helps a lot on the popular piercing bonus Colibri camps. You'll want to have the puppet melee to gain TP instead of hanging back like a RNG, because damage is more skewed toward WS. Kind of like a circa 2004-2005 RNG, or a COR meleeing with a multihit weapon to fire off as many highly damaging Slug Shots as possible. It is common to get Sharpshot for your first new frame at level 10 both for its usefulness in party, and because it's the only frame that gets ranged skill (so if you wait around until level 20/30, it's further behind on ranged skill and harder to skill up to near cap).
Spiritreaver is the BLM and is indeed often used in endgame situations, generally for stuff that's either magic-weak, or where melee is simply too dangerous. It can put out massive damage too, but it's a little more finicky to manipulate with maneuvers and to get the best performance you'll want a couple of the expensive attachments (Tranquilizer and Condenser), pet magic+ gear (Relic legs/feet, ACP body, random augmented stuff), merits (5/5 pet magic skill gets access to 3 new spells, Optimization gives a big pet MAB boost). The fact that the spells that make it worthwhile are late level is another reason you won't see much of SR until the endgame levels.
You'll still find situations, many solo or small group, where you'll use the "RDM" frame (Stormwaker) and WHM head (Soulsoother). Soulsoother can cure party members, the only puppet that can do so.
Valoredge (the melee puppet) has its uses too. It's a melee DD with high HP, great for kited fights, solid DoT, and it's more sturdy than the comparatively weak Sharpshot. Sometimes if AoE is an issue VE is the best.
Harlequin is the starter frame, kind of a jack of all trades, master of none. It's generally very rarely used outside of beginning levels. It does blunt damage, so every once in a while you'll find an oddball situation where it's useful for that reason.
#3 It sounds like PUP pretty much gets the shaft in the end-game, similar to BST?
Depends on the people/event. Some LS leaders don't understand what to do with PUP or haven't seen many good ones, so they avoid it. Some more openminded people give it a chance, and with a good PUP they'll often be impressed. Some events PUP is good in are Limbus (one of my favorite Limbus jobs), Dyna (either as another DD, or nuking stuff with BLMs), Nyzul (excellent for floor climbing, not quite as good for bosses). And of course, stuff like sky/sea PUP does fine - one notable use is that Valoredge is a BEAST on a kited Kirin fight.
Then again, some LS leaders won't do anything without cookie cutter setups.
#4 Wow that's a spendy item...is that really necessary? I couldn't even justify that for WAR, where it would arguably be even more important.
PCC? Not really necessary. I used Spike Necklace -> Chivalrous Chain myself before I ever got a PCC. The point was correct though that PUP has a bit less access to good DD gear in some slots, so where we can get really good pieces they're often even more valuable to PUP than to other jobs. Still, this concern is a little bit alleviated these days now that PUP just got a considerable bump in H2H skill to B+. Used to be we were especially accuracy starved during the mid-levels, but the skill raise helps that a lot.