I think the first upgrade is to buy a record cleaning machine and some Art D'son fluid.
No point in doing any upgrades and then playing uncleaned records...
Then Cartridge, even a 30 quid At95e will sound FAR better than the DJ cart it probably came with.
Next up, rewire the tone arm, warning - this job isnt for the faint hearted
The stock arm is fine btw.
Most people who say to get another arm maybe havn't heard a rewired stock arm, especially with a Kab tone arm damper fitted.
Stock arms often sound crap because the wire Technics used is rubbish, and on a 20 year old arm its gonna be oxidised badly.
Next mod, get an Ian Mac bearing, massive improvement !
Then, get a decent phono stage, I recommend a secondhand LFD Audio MMO, sounds lovely !
Then after that, the mods only make more subtle differences, Mat, Feet, power supply...