I want to put together a fairly comprehensive toolkit to work on both vintage and modern bikes (I just started buying bikes on CL, fixing them up and reselling them). These are the tools I have so far:

Tire levers, air pump, 15mm/pedal wrench, adjustable wrenches, allen keys, cable cutter, chain tool, spoke wrenches

This is what I'm considering getting:

Chain whip, cone wrenches, crank puller, bottom bracket wrench, fourth hand tool.

Anything else I should be getting? I'm probably going to buy from niagaracycle.com, they seem to have the best prices and selection.

You may want headset wrenches, but for many older bikes just a large adjustable wrench may work well enough. Also important for working on older bikes specifically are small box-end wrenches in 5/16", 6mm, 8mm, etc for small cable anchor bolts. It can be difficult to get these properly tight without stripping with just and adjustable wrench.

Many older bikes also have different style bottom bracket tools, but for many non drive side cups a track type lockring spanner will work in combination with a pin spanner. The fixed cup usually needs the correct wrench, can't remember the name.

As for supplies, make sure to have brake and shift cables, housings, and brake pads. Replacing those parts on an old bike is often the most worthwhile thing you can do.

Fixing up used bikes from CL and selling them can be fun, but its very difficult to make money doing it.

Other than basics you can anticipate, the smartest way to buy tools is to wait until the need comes along. Don't worry, you'll accumulate more tools that you can imagine soon enough.

Not sure the 4th hand tool will be nearly as essential/useful as the others. If you're dealing with disc brake rotors, you'll need a T25 torx bit/driver as well.
I'm guessing you already have chain lube and (hopefully) a good grease. If you need a cheap good grease, this stuff usually costs about $5/tub at your local home improvement big box store:

My garage is really small, and my wife's minivan fits inside. And I need room for my fishing gear and rod-building hobby too, which I spend more time on than cycling. But have most of the above tools listed, but I'll add:

- freewheel remover tools (freewheel vise, 2-pin, 4-pin, splined old shimano, and splined new shimano cassette tools).
- truing stand and dishing tool (portable so I can move it around - not enough space to clamp it to a surface)
- a set of metric and english taps and dies
- traditional threaded headset tools (cup removers, crown race setters, cup installers)
- star nut setter
- sealed cartridge bearing extractor
- about 3 or 4 sectioned tackle boxes or oil proof hobby/sowing boxes for lots of screws, nuts, caps, ball bearings, spacers, washers, etc.
- measurement tools - set of good calipers, tape measure, yd long level (all in dual inches and metric)
- degreaser, rust remover, polishing compound and soaking bucket
- grinding wheel and buffing wheel.

optionally - some expensive stuff
- shop stand with clamp
- spoke threader and or a threader/cutter tool.
- BB face and chase
- Headtube face
- crown race cutter,
- 1" x 24 tpi and 1 1/8" x 26 tpi fork steering thread cutting dies,
- spoke tensiometer,
- frame-fork alignment tools,
- MAPP gas torch for small brazing operations
etc.

I bought this:

https://www.amazon.com/Park-Tool-AK-3...7965175&sr=1-2

It's good for a start. I imagine as I get into more complicated work I'll be supplementing the tools.

Yeah right, this guy is flipping bikes already without tools, like he's gonna try brazing.

Higher margins on custom-stuff... In college, I had a bunch of Bridgestone 400's. Decent fork and chainstay clearance for 700x32C tires (was originally 27 inch wheels). Some buddies wanted training CX bikes. Found a source for cheap cantilever bosses and got some dremels, wire brush, flux, solder and torches. I brazed a few, and learned some secrets on making strong bosses that don't crack off on the first squeeze of the brake levers. Some cheap spray cans of paint and good taping jobs and lots of clear coat, and we had great frames that looked totally awesome. I still have an unpainted fork with bosses soldered on.

What's wrong with taking a junker bike off Craigslist for $50, added $50 in parts and lots of TLC and maybe even brazing and some paint? Turn around, and sell the thing for $300. That's 66.7% margin on something that you might put a couple hours working on if you're prepared to do that kind of work and fast at doing it?

IMHO the spoke tension meter is a necessary tool. I can't tell how tight exactly a spoke is by just feeling it. It's nice to have it while doing quick maintenance work on my bikes. When I run into a loose spoke, I am able to quickly get it to the same tautness as the spokes around it [or on rear wheels--the same as the other spokes on the same side].