It's always iffy to state how "hard" it might be for someone to build what is a pretty straighforward amp.
The hard part is not so much the wiring; either based upon the schematic or what many people prefer, a physical layout diagram. The development of a layout dwg (from the schematic) is trickier than it might appear, but at least you are just burning up pieces of paper instead of soldering parts. Having done this for many years, I rarely used a layout dwg, relying upon the schematic. The biggest issue with EITHER approach is that if you leave something out, it is very very common to find you have backed yourself into a corner and created a mess, or, have to redo things.
The harder part, to me, is the metalwork. That is assuming you do not have many of the tools needed to work sheet metal. It is hard to get your first, second, and third try very neat.
Your idea of salvaging a dead SS amp, gutting it, and building inside same is completely excellent. Go find a dead Peavey you can buy for $25 and you might get a speaker in it. Super! It is not easy, but it is no harder than building on a raw chassis and the good part is, you end up with a cabinet with tolex covering and a handle and corners which is about $300 grand total if you have to go buy it. And if you are successful building a functioning thing, you can move it around. Much better than a voltage-exposed science-fair project. And you may get a pile of knobs and jacks in the bargain. It is easy to underestimate the total cost of all the spacers and jacks and knobs, every one of which costs $2-3 if you have to buy them new. And getting that odd part or ten that you forgot is time consuming.
Again, having scratch-built amps for many years, the number of times it has gone really smoothly that I can recall would be "zero". There's always something. Even for me, with bucketloads of old parts lying around. Sometimes you just have to spend more time & money than you'd like. Sometimes you do that and you STILL end with a fragile thing that you can't really bring anywhere outside of your house because it's fragile and an unenclosed shock hazard. Or something you just cannot get the hum out of because of the way you built it.
So without trying to discourage you in any way....it's easier and in the end no more expensive to buy a kit, esp for your first project.
It's the metal that IMO is the big deal.
Spend some time wandering around this forum and you will see every variation of neat, sloppy, difficult, easy, and some absolutely beautiful examples of custom built amps.