In years past they certainly were useful, so recent ones might also be.
Reading your question, I realised that our one is probably 35 years old now, and still running fine. Like yours it only gets used occasionally, as we have a walk-in airing cupboard where the boiler is, so it only usually gets used for towels and other things that need to be "fluffy" dried (if you see what I mean!).
Basically it has a simple reversing system: ours I think is relay logic (the switches are relays, with a timer circuit to activate them). The drive is a long belt that goes around the drum, and is tensioned by an idler wheel. I don't think the main motor does the fan, but I might be wrong - it's decades since I had the lid off, probably. IF it does break I shall repair it...
... so the motor is simple and reversible. This is good. It's also an induction motor, so it's quiet, but whatever you make will be single-speed unless you do something complex like add a variable frequency drive (which sort-of defeats the object).
Myfordman, of this parish is the expert, and can advise on its wiring, but I think it most probably can simply be wired to run in one direction on a switch. I'm fairly sure the topic has come up before, so use "search". Also there is at least one Haynes DIY manual on washing machines and dryers, which might give you a good idea about the innards without having to dismantle it first.
The machine likely has a flat belt, and a corresponding pulley on the motor, which isn't ideal but is usable for other tasks - you might need to get creative, and a matching pulley for the disc sander end will be harder to find. So although you might stick with the flat belt, a replacement v-groove pulley set would, allow you to use a belt of your choice.
I would expect a traditional v-groove belt would grip better at lower tensions. Again, traditionally, the motor is mounted in the pedestal of the tool, below the business end, so that gravity can aid you in tensioning the belt. It's also far easier to fit a long belt than a short one.
The other reason for swapping the pulley is the gearing - obviously the motor's rotation is geared down hugely for the drum (probably doing only 30 RPM or slower). You want to go the other way, I expect, to get your sander running at a good speed. You can't alter the motor's speed (without a VFD), so different pulleys are the only way.
At the sanding end, you'll need a shaft, pillow blocks and another pulley. It could work out to be expensive, but the finished unit will probably be stronger than one bought off-the-shelf. And if the motor does die eventually, you can simply fit another one.
All that said, it sounds like a fun project!
