I have a Rega P3 2016, I specifically wanted this one as it’s the first model that you can really modify with some stellar components to elevate it higher.
In my journey, these are the things I’ve found have made the biggest impact:
Subplatter: Going from the standard P3 plastic sub platter up to a GrooveTracer Reference subplatter with sapphire bearing was a major improvement.
Isolation feet: swapped out the standard rubber feet for some ISOAcoustics Orea Bronze, had problems with footfall on the floor, and this just eradicated that.
Cartridge: stepping up to an MC cart had a major impact, less surface noise, beautiful mids, punchier bass. Better separation
Phono stage: often gets overlooked, but incredibly important to the sound you’ll get out of your table.
Neo PSU: not certain I can discern an audible change here, but for generral ease of use switching from 45s to 33s, it’s an absolute must imho.
And as a side note, I had a custom lid made as I didn’t like having the included lid attached and open, preferred it with the lid off, felt there was some vibrations with the lid attached.
Out of all that what did I learn:
I’m really happy with my system, and it was fun making each upgrade, but if I did it all over again, I would plump straight for a P8. With all the mods, I probably doubled the cost of the turntable, most of the components are included in the higher end tables.
And yes, as @RonP says, condition is all important, and discogs can be a minefield. NM is the lowest I’ll go and I’ll spend months hunting for the right record. Somewhere like EIL where you know they will only supply NM condition unless otherwise stated, and their reputation preceeds them. Not sure if you’re UK based or across the pond.
https://eil.com/shop/presetlist.asp?PageId=865