So this is my first guide, so if it's a little messy bear with me.
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Summary:

So this build is entirely around being completely and utterly rich beyond your wildest dreams.  In doing so, you can afford the biggest ticket items essentially whenever you want, and can likely change on the fly based on your circumstances end game.  See too many attack armor heroes, switch to starheart or something instead of deathstroke's claymore.  The possibilities are endless!

This build is also a "racecar" build, in which at end game she'll nearly break 400ms base and improving way beyond that when the bullets start flying.

With this build, I typically end the match with a good 3-4k gold advantage over the 2nd highest player in the game, and in many matches I buy and sell items wholesale from the vast amounts of credits I'm sitting on end game.

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Mods and Amplifiers:

Altered Olympus/Qward/Oa Gambits for Investment 2 Keywords.  

The reasoning?  1.5 credits a second.  It's an insane money generation that allows you to let your carries last hit if you're duo laning, or if you feel like doing so, to just make even more insane amounts of money while also last hitting.

Auguments:
Codenames:
Portal (PD/level (G) & AD/Level (B) )
Gank (AD/level (G) & PD/level (B) )
Horsepower (Attack Pen (G) & MS (G) )
Samurai (Power Armor/level (G) & MS (S) )

Harley has hybrid damage on both her q and her r attacks, and she is quite slow to start with.

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Item Build:

Start with the Qward Gambit.  No potion?  NO.  You can pick it up if you're getting harrassed early easily with the extra generated gold, but is unnecessary, as you'll be spamming heals most of the early game on yourself and allies, not to mention the healing stolen power.

Then get the other two, the order doesn't matter but if you're facing a PD harasser get the power armor one first.

With these three items out of the way early, you're now going to start rolling in the credits.

*Note:* The "credits earned" tooltip is bugged and will only show the money generated from a single investment item, but they are stacking.  It only takes a cursory look at your credits spinning like a cartoon to see that.

Next, focus on Pandora's box.  This allows for your carries to chase, and also allow you to chase/escape as needed due to the movespeed buff.  Also, the CD reduction allows you to heal even more often, which you should be doing nearly every cooldown.

The next item is up to you.  I like going for dreamheart for more lane sustain, but if you're really stomping the other team you can begin working on the implants.  Either way, when you get one, immediately build the other.  Now you have a near infinite source of will, a 35% cooldown reduction, and a great attack speed.

Now it's time to bring the pain.  Grab Joe Chill's revolver and then finish up with Lobo's Chain and finally Deathstroke's claymore, selling the investment items one at a time as you build each weapon.

If the game is going well, you should be able to afford all these weapons by the 30-40 minute mark, in which case you would have earned nearly 2000-3000 bonus credits from your investments.

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Situational Items:

The situational items listed are if you decide you rather go a more PD damage route in case they have some of the more duable AA heroes, or a change of pace.

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In closing, enjoy, and put the fun in FUNeral!