Lower receiver, check.
Upper receiver, check.
Barrel, check.
Forend, selected but not yet ordered.

How many parts do I have to order ahead of time before I can walk into a gun store and say "Hey, sell me what I need and help me put this together?"

I want to spend some time selecting a stock, because honestly, I have yet to shoot an AR with a stock I like.  And I have some particular desires about the sights.  But the rest?  I just want it to work reliable and not cost a ridiculous amount of money. 

Or, to ask this another way, how much do I have to prebuy before I can I reasonably expect to walk into a gun store and say "Hey, Tex, sell me enough parts to make this work, and let me help you assemble it?" 

I'm not too sure what our LGS carry in stock as I got most of this stuff from amazon or Rainier Arms. I'll note where I got something locally.

BCG
Pistol grip-LGS
Gas block-LGS
Charging handle-LGS
Buffer tube, spring and buffer
Lower parts kit
Gas tube
Trigger group [ If you're not going with what came in your LPK ]
Sights and/or scope
Stock
Muzzle device and crush washer
Barrel nut if your forend doesn't come with one

Hope this helps! 

I think edster has it covered.  I would say of all those items, probably the grip and stock you would want to buy from a LGS or at least look at them before you buy as they're user preference.  Maybe the trigger too but you can probably use the one in the lower parts kit for now until you decide what you like.

Lower receiver, check.
Upper receiver, check.
Barrel, check.
Forend, selected but not yet ordered.

How many parts do I have to order ahead of time before I can walk into a gun store and say "Hey, sell me what I need and help me put this together?"

I want to spend some time selecting a stock, because honestly, I have yet to shoot an AR with a stock I like.  And I have some particular desires about the sights.  But the rest?  I just want it to work reliable and not cost a ridiculous amount of money. 

Or, to ask this another way, how much do I have to prebuy before I can I reasonably expect to walk into a gun store and say "Hey, Tex, sell me enough parts to make this work, and let me help you assemble it?"

the stocks and grips you should check out at the HDF range day

If you're asking about availability and not price, I've found a decent selection of all parts except barrels. SEC, KGS and WGS have a good selection. I don't get out to X-Ring, but I think they have a good selection also......Speed?

My go to sources online are DSG ARMS and Brownells.  I do use many other places on occasion, such as Rainier, Bravo Company, Midway, etc...  DSG and Brownells can be one stop sources.  I do understand that purchasing some parts local might get you some added assistance in putting the weapon together, but it would probably be less complicated to order everything online.  I also get the supporting local business thing also, which I do, but some things are better left to source online.  There are places or individuals that would probably aid you in the assembly process, maybe for a small fee, or maybe even for nothing.  I am sure you can get some advice on who to go with or who to avoid.

I'd recommend buying your lower locally. I bought mine at x-ring. Almost everything else I got online. Took awhile cause I'd wait for deals or sails. AR15.com is a wealth of knowledge for building

WGS have been very helpful since they started in 1997. Prices are a bit higher but I'd support them when I can. Humble and helpful. Two great traits I always look for  Too bad they are not on this forum.

Unless the gun shop/FFL has a manufacture's license, they can't help.  Giving advice is one thing, but it's not legal for anyone but the person building the gun (for personal use only, not for sale) to do any of the assembly.

Unless the gun shop/FFL has a manufacture's license, they can't help.  Giving advice is one thing, but it's not legal for anyone but the person building the gun (for personal use only, not for sale) to do any of the assembly.

Too true, and I don't know of any shops here with a type 7 license.

Best to get a knowledgeable friend to help, maybe ask Kendra?   

Unless the gun shop/FFL has a manufacture's license, they can't help.  Giving advice is one thing, but it's not legal for anyone but the person building the gun (for personal use only, not for sale) to do any of the assembly.

Wait, what?

I've had more than one gun shop owner offer in person, and seen numerous cases on this very forum of them offering that very thing!  Why would it be illegal?  I already own the registered lower, which (it seems to me) would be the only piece requiring a Type 07 to make. 

None of the following is legal advice and there is no claim of accuracy in any information in this post.  Consult with an attorney, or the ATF, or both. 

If a company has a customer that provides their own receiver and parts and then the company does the assembly for the private individual, which was for that individuals own personal use and the work was not done for the purpose of sale or distribution, the company needs to only be licensed as a dealer or gunsmith.

If a company buys one receiver and then builds a complete rifle and sells the one rifle, not as a regular part of business or trade, it is not considered a manufacturer.

If an individual assists another person in the assembly of their own weapon for personal use and it is not a regular part of business or trade, it is not considered manufacturing firearms. 

Selling or even providing knowledge for free is not illegal.  There are those who might "aid" by providing you guidance while you did the physical work yourself.  They can be present throughout your entire build.  Heck some might even "rent" or let you "borrow" the tools also.     

technically, they are correct a shop cannot assemble a complete firearm without pay a tax.

but nobody really cares 

Here's a quote fron the BATFE FAQ on the matter:

A company receives firearm frames from individual customers, attaches stocks and barrels and returns the firearms to the customers for the customers' personal use.

The operations performed on the firearms were not for the purpose of sale or distribution. The company should be licensed as a dealer or gunsmith, not as a manufacturer of firearms.

That link was what I was looking for.