First is the truck a double cab or access cab?
Nothing wrong with doing the upgrades in stages as you suggest, just depends how many times you want to tear you interior open.
1. Sound deadening/proofing.
There are many retailers whom offer an assortment of products. I'll focus on the 3 most common: sound dampening mat, closed cell foam(CCF), and mass loaded vinyl(MLV). Sound dampening mat kills resonances/vibrations in a vehicle but does not sound proof. CCF acts as a decoupler, ie separates the mat from the MLV. By itself it only absorbs certain higher frequency ranges, most of the road noise, exhaust drone, occurs in the lower frequency ranges. The MLV is where the true sound proofing takes place. The MLV sheets weigh roughly 1lb per square foot, and the installation has to by a solid wall of material to be the most effective.
Most people use sound dampening mat and are satisfied. Ideally you would use all 3 products in a sandwich one on top of the other. Sometimes that doesn't always work with space constraints of the interior. Depending on how far you would like to take this step, you can do it cheap or all out, all out gets really expensive with quality products.
never used their material, but seems to be the standard that everything is compared too.
Rick is great guy, offers an affordable product that does the job, the material is easy to work with but a little messy, plus tacomaworld members get a discount
top of the line products, they have everything you need in one stop but bring money, i'm going to use this retailer for my wife's 09 taco at some point, my 99 ranger not so much
the owner of the site, Don, used to test various sound dampening products and now offers his own products, the site is very educational and is the place I'd start to learn about what all the different materials do
Mat to avoid FatMat, used it in my wife's old F150, shit fell off, smells horrible and customer service was non extistent.
2. Power cable, RCA's, speaker wire
I've used stinger in the past, works well, no problems. I bought the cable locally so the price was a little steep. Some people on the board have used welding power cable in the appropriate gauge for their installations. The colors aren't as bold(bling) however the cable seems to be flexible enough to do the job, which can be a real concern. I recently purchased an amp install kit from www.knukonceptz.com
nice product should work as advertised. They also advertise most of the hardware you'd need to install amps, speakers, and head unit RCA's.
Speaker wire is up to you. You could buy a roll of 16 to 12 gauge speaker wire from a home audio store and it will do the job. Smaller than 16 gauge and the wire becomes difficult to work with and 12 gauge can be a bear to stuff through door bellows, and is probably overkill for component speakers.
Any quality RCA cable will do the job, I bought into the crutchfield hype about StreetWires RCA's. Probably could have spent less money for the same outcome. Again www.knukonceptz.com
offers various levels of RCA's for reasonable prices.
3. Head units
Personal preference, decide what you would like to do with your system. Do you need the HU processing to adjust cross over points? Are you going to run your system active vs passive? Out of all the stages you listed this is probably the easiest to do. You could probably get the install done in about 8 hours from start to clean up.
Personal preference and how much you want to spend. Space constraints come into play big time with amps, especially in our trucks. Mr. Marv a vendor on this site, has made some beautiful amp racks and sub enclosures that fit behind the rear seats of the double cab. There's a whole laundry list of amps to choose from, do your research and set a budget. I'd install both amps at the same time. If Mr.Marv is building you a custom enclosure/rack the subs will go in at the same time.
5. Component speakers
Every ear is different. If you've run all the wires in the previous steps, installing the speakers in their stock locations will be easy. You can make a shitty speaker sound great in a proper installation and a great speaker sound shitty in a poor installation, install is 90% of the work. Passive components are good for most folks, but if you like to ticker going active properly tuned will give you a better sound being able to pick and choose individual speakers/drivers, this is why item #3 is so important.
is a great site that discusses everything car audio and then some. Read all the tutorials and build some knowledge before spending mucho money. I've learned a great deal in the few months since I've joined the forum.
It sound like you have the time to research a little more, no impulse buying, which is good. The steps you've laid out seem logical, but like I said earlier you'll get to know the interior of your Taco very well by the end.