I know nothing about brifters, other than what they are.

A friend has come with a drop-bar bike and asked if they can have brifters. It's a Norco Monterey with a Suntour R rear mech and a six-speed block.

What does she need - we're a looking for low cost solution.

the suntour mech won't index with shimano brifters, nor will the 6spd freewheel.

as far as new parts, you'd want an Altus RD-M310 derailer, Tourney STI shifters, and a 7 speed Tourney freewheel (which will thread on the hub but may require an extra driveside spacer, possibly a longer axle, and a redished wheel to center.
Or, a cassette wheel and a 7 speed shimano cassette (or 8 speed, or 9 etc speed everything parts)

For used, most 7 or 8 speed shimano derailers would work. As would any old RSX or whatever 7 speed STI shifters you can find. It has to be 7 speed if you keep the screw on cluster wheel as you can only get a 7 speed freewheel with the correct shimano spacing on there.

Honestly it doesn't make that much sense. I would suggest some friction bar end shifters, or convert to cruiser bar with some thumbies or a different bike.

As above, but I would go with Microshift brifters. I like the separate upshift/downshift paddles.

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The criteria for indexing is to have the "pull" of the shifter match the movement of the RD that has to match the spacing of the rear block.
Then the issue with making it work is the spacing of the rear over lock dimension between the drop outs to accommodate a 7speed block on the hub. Then it is about chain line for good shifting.

Explore the Sheldon Brown site. you might start here: 6-speed, 7-speed, 8-speed, 9-speed, 10-speed, 11-speed? (sheldonbrown.com).

If the Norco has a RD hanger I'd find a donor bike and take everything of and put it on the Norco.

Personally I don't get all the hubbub abut 'brifters' but perhaps I don't ride fast enough or in hilly enough terrain. For my riding style style 7 or 8 speed with Dt indexing or friction is OK

To get 7sp, you need new shifters, new cassette(freewheel), new rear derailleur, new front derailleur, new cables and housing, and new bar tape.
To get 8sp or higher, you need all the above plus a new rear wheel.

As mentioned, Microshift will be the cheapest STI option. You can get 2x7 for $45-65 on ebay, depending on timing. Plan on this costing at least $150 at 7sp with entry level components.

I get that. I was skeptical until I tried them. One of my goals is to get my RPM up. I find that the Ergos, in my case, cause me to shift more often, keeping my cadence up, and can do so without much planning ahead. there are times when I need to react quickly and DT's don't always give me the option to change gears during the reaction period. Not a big thing but I like the added flexibility. The bigger hoods are nice too I now ride a lot on the hoods. Non aero campy hoods are a bit tiresome over time.

I read a lot of people have a fear of letting go of the bar but that comes with time and self confidence. If you ride a lot, there is no fear of one or no handed riding. There is a technique to riding a bicycle effectively, many haven't learned that yet, like who steers with the bars over 10 MPH?!

I've been using brifters exclusively for 25 years now, in fact I think since taking up cycling again at the age of 38 I may have ridden only a couple months with downtube shifters. But last month I got an itch to restore my long-neglected 1994 Cannondale to something like its former glory. I still have 8 speed cassettes, and I still have the original aero brake levers. Everything else, I had to scour Ebay for. Right now, it's having the Campy 'Racing T' triple crankset and BB I added swapped out for a Shimano RX100 double and appropriate BB. I have the original SIS shift levers, but they were for 7 speed, so I have an 8 speed SIS rear lever on the way. I can't wait to get it all built up!

You're right about not having to plan ahead. I find with brifters I am constantly responding to load and cadence and keeping it all in a fairly narrow range. Mostly I don't even have to adjust the pressure on the pedals while shifting, just *click* and it's in the next gear. On Strava, my cadence graph is almost completely flat, with sharp drops where I stop pedaling. I'm really curious to see what happens when I ride with DT shifters.

The OP is looking for a low cost solution. There is no need to buy all new parts. Basically all 7 to 10 speed shimano road rear derailleur (except for Tiagra 4700 and Dura Ace 74**) will work for this application. As for the freewheel- keep it 13t or smaller for the small cog. A 14t + small cog could interfere with chain stay. Avoid the DNP one though since that one is extra thick.
By the way, if the 7 sp freewheel fouls the chain stay, all hope is not lost. As a last and not so cheap resort, you can run a Suntour 'Ultra' 6 speed freewheel. This 'Ultra' 6sp has the same cog spacing as the 7 speed freewheels, so they are compatible with the 7 speed brifters. You will have 6 gears to work with, and an unused click on the brifter.

What exactly is a Brifter?

BR (ake plus sh) IFTER - I think most specifically to the manufactured varieties of these for drop bars, not the various hodge-podges of shifters-stuck-somehow-to-brake-levers that we garagistes have come up with.

Using a donor bike for the parts is definitely the most economical solution, compared to collecting various used or new parts yourself.
Participating in a local bike co-op may allow a greater spectrum of available cheap(er) parts and/or candidate donor bikes.

They're a difference maker for me, especially with an 11 speed rear cluster. As much as I love old bikes parts, any bike where I'm going to have extensive and frequent saddle time will always have brifters and at least 11 speeds.