I've often "up-cycled" enclosures from consumer gear, but in practice, it's not as easy as it first appears. Replacing the front panel can be difficult because the bottom/top/sides usually slot into it for rigidity, and there isn't always a clear way to attach the new panel. Sometimes, retaining the old front panel but "re-skinning" it with a new sheet can work well, but that relies on the original being largely flat. For something like a hi-fi amplifier - which has almost no controls (thanks to audiophile "minimalism") - you can can re-use the positions the old controls occupied.
Another problem is that the steel can be incredibly thin and hence hard to work with - as counter-intuitive as that sounds...
Old test gear is usually more promising, but of course old test gear does tend to be more fixable and hence more valuable. IT equipment - especially the more professional kind - can also be very good. Think about hubs/switches, modems, terminal servers, UPSs, etc. Nice rectangular steel boxes that are well-shielded and solid.
But yes, enclosures are expensive - especially from the big suppliers like RS, Farnell, etc. Whenever I rummage through the skip, I always carefully check anything that might yield a useful box, whatever it is.
Of course, the DIY route might be feasible - e.g. a simple MDF surround and aluminium front/rear panels. If you need to screen a wooden box, aluminium tape is often available at Lidl really cheaply (very handy stuff!). Aluminium extrusions are very handy, and I always rescue them when I can (I use Aluminium Warehouse
if I need to buy something). For example, angle that is 2" by 1/2" makes for good front/rear panels, which can be attached to a sheet of aluminium that forms the base, then followed with 2 bits of wood for the sides and top. If you like, you could use a sheet of perspex on the front panel, held to the aluminium angle by the controls and connectors - then, a sheet of paper with the control legends could be sandwiched between the perspex and aluminium. If you're feeling especially swish, you can even back-light the legends...
Last time I did any of this sort of stuff, I only had access to basic hand tool, plus a jigsaw (which surprisingly cuts sheet aluminium better than timber), and a woodworking router, which I found worked especially well with aluminium provided you go slowly. It's time-consuming, but at the time (pre-kids!), that was part of the fun.
Anyway, hopefully some good general ideas in here. As far as I can remember, I used one of these
and mounted the controls on the lid. It was a bit of a squeeze, but it all fitted. For the PSU, I used an RS PCB, but is was smaller than one of the ones you have, and it generated both rails from one chip. The transformer was a 6VA frame type. But no space for a power amp or anything else - I built the amp in another box of the same dimensions.