I would start with speakers. If you choose right you will enjoy them for a long time. Then power amps and speaker cables. This would make a speakers/power amps/speaker cables unit complete. After that - preamp to match the amp but also future phono stage and dac.
I would upgrade turntable/tonearm/cartridge last, this step is just as important as speakers. Tables/arms can last for decades.
But if you are going to buy used you will be limited by what's available at the moment, so it might be more chaotic and complicated. Ideally, you want to have the entire signal chain in your mind before proceeding. Still, speakers first, unless perhaps you want to use particular esoteric lower power tube amps and then match speakers to it.
I would upgrade turntable/tonearm/cartridge last, this step is just as important as speakers. Tables/arms can last for decades.
But if you are going to buy used you will be limited by what's available at the moment, so it might be more chaotic and complicated. Ideally, you want to have the entire signal chain in your mind before proceeding. Still, speakers first, unless perhaps you want to use particular esoteric lower power tube amps and then match speakers to it.