We've use unpunched off-the-shelf chassis and drill/ punch the holes.
Whilst this is cheaper than buying a prepunched chassis and gives you greater flexibility, it is time consuming, and you will need to buy some chassis punches (we've got Q-max cutters) and/or cone cutter/ s and some decent drill bits, which might make the exercise more expensive then buying a one off. The cone cutter works really well and deburrs the holes.
Having said that, once the initial novelty of cutting nice holes in a piece on metal has worn off, I find this a bit of a pain, especially the deburring. Cutting the hole for the IEC socket is a pain with out a chassis punch (drill/ filling etc), and the IEC chassis punch is expensive.
Re choice of metals, steel is stronger, heavier and cheaper than aluminium, but is harder to work and will need some form of treatment to prevent corrosion. Steel is magnetic, so will conduct flux leaking from the mains transformer. This could be deemed beneficial as it could shield the small signal regions of the amp from any magnetic interference, however you could also be argued that flux could be conducted to the small signal regions of the amp and induce hum into the signal path.
Aluminium is non-magnetic and will not conduct flux through the chassis, so avoiding leakage flux inducing hum into the amp is simply a matter of ensure the input stage is as far as possible from the mains (which is good practice anyway).
Stainless steel is very hard to work, am I would not recommend it as DIY project unless already punched.
Low noise, great sounding amps have been constructed using all the above metals for their chassis.
We've now ordered a batch of our own chassis for amp construction on 1.6 mm steel.
For the electronics, we've use tag strip, tag board, eyelet board and turret board for amp construction (not all on the same amp I hasten to add!).
Tag strip requires some care regarding the layout, is not that flexible layout wise and can be difficult to modify as the components should ideally be crimped onto the tags. I've also had problems with DC leakage on tag strip, so I would also ensure that high voltages are connected to tags as far away from small signals as possible. Another problem with tag strip is that the tag strip has to be bolted into the chassis before construction can start and this can make assemble more difficult.
Tag board is probably easier to use than tag strip. I've not had DC leakage problems with this, although as it's made from the same material as tag strip this could be an issue. Tag board can be assembled outside the amp.
Turret board probably provides the sturdiest method of amp construction as a very good mechanical joint can be made before soldering. You can buy board with turrets attached, or make your own, which gives good flexibility of layout. Of the methods listed herein turret board is the most expensive method of construction. To build you own boards you will need a tool for swaging the turrets in.
Eyelet board is cheaper than turret board, and if you make your own is very flexible in the layout. Making your own eyelet board is easier than making turret board. Having repaired tons of Fender amps, this is a very reliable method of construction — it is very rear for an amp to have a fault due to a problem with the board — and changing parts is very straight forward, which should be a consideration if you want to tweak your design.
I now favour a mixture of turret and eyelet board, as I think that this gives the best flexibility regarding layout, cost (eyelets are way cheaper than turrets), and sturdiness (turrets can be used for wire connections off the board).
Regarding board material, FR4 is very good, but we now favour 3 mm PTFE, although 1.5 mm PTFE works OK too in Fender style amps. PTFE has better electrical properties than FR4 (or indeed anything else), is very easy to work, and doesn't produce nasty particles when drilled, unlike FR4. It also costs about the same as FR4.
We use relays for switching. Optocouplers are relatively expensive, unreliable, noisy, and not great switches as they can have significant feed through in high impedance circuits. JFETS can also be used as long as attention is paid to the maximum signal then will see.
