Designing a buck converter circuit
Application notes are how you go about learning how to design anything electronics related in the advanced cases where no set book exists and would usually require listening to a boring lecturer yammer on for three months without ever actually managing to teach you how to design them from the ground up. For example, you will hardly find a good explanation on how to design a Variable Frequency Drive for an AC motor or why it works other than that it is a fancy power inverter. You might find some vague schematic such as what you provided, but not much other than that. Microchip, for example, has some really extensive information on the subject that you simply would not find anywhere else. You usually won't find these app notes with a random google search, you will have to go on their respective websites and search there.

International Rectifier has, or had, an appnote with a section on creating a 15V SMPS with 40-350VDC input, of which you can simply just use a full bridge rectifier and filter caps and some fuses and power it from rectified mains AC. Appnote 1044, page 15. If you were not intending to use it for powering a laptop and hard drives the design provided with the IR2153 as is should be acceptable for testing and learning about buck converters/SMPS, you could then just use buck modules from TI or wherever for your other rails, personally my favorite being the LMZM23600, although it does not meet your current specifications. If you want to modify the design in AN-1044 to supply more current, you will undoubtedly need a different inductor and a better MOSFET than the IRFR420A. You should also follow up with the IR2153's datasheet as well as the appnotes suggested on Infineon/IRF's website for the IR2153. 

The 2153 can be considered to be a 555 timer (your PWM clock) with a gate driver (your switch) attached, so if you are familiar with those then it shouldn't be too hard to learn about buck converters/switch mode supplies with the 2153. If I recall the schematic correctly there is a programmable voltage reference IC that controls the output voltage. It might not be an appnote on SMPS in particular, but it certainly does provide you with a circuit you could use/screw around with to learn about how they work. You can also use Analog's LTC4367 as a hysteretic voltage regulator as noted in it's datasheet, it uses less parts although you certainly couldn't hook up mains rectified AC to it. As far as how they work, in short, SMPS's are just gate drivers with an oscillator that defines the frequency of a PWM signal, where the duty cycle of the PWM wave defines the output voltage and is modified by the control/feedback circuitry as the load increases/decreases or as the voltage increases/decreases. The LTC4367 is also similar in that it switches on and off rapidly to generate a stable voltage, except that it doesn't use an inductor, just two power MOSFETs and a few resistors/capacitors. It's switching is just performed through the 25mV hysteresis of it's comparators, which then switch the MOSFETs on or off.

As far as heat goes, as long as you use low RDSon MOSFETs, you shouldn't have much trouble, if any. There are appnotes on calculating the heat dissipated to ambient and to the case/thermal pad from Texas Instruments and Maxim, you can google around for those. If I recall right one of the equations was (TjMAX - TjAMB) / (Thermal Resistance), IE- (125-25) / 83 = 1.2W, so you can dissipate 1.2W safely without a heat sink, and if your MOSFET has an RDSon of, say, 23mOhms, I = sqrt(1.2W / 23mOhms) = 7.22A since I^2 * R = P. You should certainly be fine with a 3D printed box as long as you use good components, have a good design, and provide for heat sinking/airflow if neccesary. You should also be aware of what temperature your plastic will melt at, it wouldn't hurt to have a comparator and an analog temp sensor drive a MOSFET to shut the whole thing down if it reaches a dangerous temperature. Your hard drives themselves really should not be pulling a whole lot of power anyway to begin with.

Just my two cents, personally I wouldn't even consider rolling my own SMPS to power hard drives and a laptop, its really just not worth the time. You could certainly do it, and you could certainly study every SMPS circuit on the web, but in the end you are relying on a design that you came up with/copied without really knowing much about the design or how it works in the first place. A lot of the schematics you find will be from some generic "free schematic" website, and some of those designs are down right dangerous. Powering a laptop from something you designed is not really a good idea, it's not worth having a smoldering/exploding laptop, you would be better off just using the adapter provided by the manufacturer, that way you can complain to them that they blew up your laptop. You could just use a cheap PC power supply for your hard drives (since they are already designed for that anyway) and whatever is connected to them, and then solder up some wires from the AC terminals inside the chassis of the power supply (preferably after the fuse....), route the wires through a hole, hook the wires up to a standard female plug, and slap your laptop charger on it. I would not disassemble any part of the power supply, too many things can go wrong. You could then simply just build a box, whether it be out of wood, metal, or plastic, it doesn't really matter as long as it has good airflow. A simple homebrew brake press, a drill, and tin snips/a nibbler is all you need to make a nice sheet metal enclosure, where you can install any sort of plug, port, screen, or otherwise onto the chassis. It might be a little jerry-rigged on the inside with the laptop charger and all, but it would only have one cord going in, and one going out to the laptop, with your hard drives protected inside. You can just as easily install plugs, switches and what not if you use plastic instead.