I have taken a stock Rega Planar 3 (2016) and added the following components (I will include the orice of same so that you can determine if these upgrades are for you):
Base REGA Planar 3...................................... $ 995.00
Groovetracer Reference Sub-Platter................$ 275.00
Groovetracer Delrin Platter...............................$ 350.00
MCM Tech Sorbothane/Aluminum Feet............$ 129.00
REGA White belt...............................................$ 39.00
REGA Neo PSU power supply..........................$ 395.00
The total investment for my Planar 3 was $2,183.00.....The base Planar 6 is $1,695.00. The $488.00 difference in price with the upgrades has helped me create what I dub a P7. Hearing them side by side, I am delighted with the upgrade path. Groovetracer is a serious company. The SIngle machined Sub-Platter and NEO PSU made the biggest difference and my table more than holds its own against any other $2000 TT available. Adding the Rega ANIA Pro Cartridge sealed the deal. At $3,200.00 with a top quality cartridge it sounds awesome and close to the P8 (which is 3995 with Ania Pro MC aboard) I was coveting, though I dislike the unfinished skeletal path and the need for a fully closed unit with plinth and dust cover was required for my kit set-up.
Ultimately, you can buy a Planar that is 90% of my table as configured and plug n play. But The path I chose was gradual and fun -- listening to the subtle changes as I replaced and added these upgrades was part of the audiophile journey for me. Other knowledgeable fellow audio aficianadoes will propose a different path. Ultimately, my next REGA model leap will be to the P10, but I am very satisfied presently.
Good luck in your REGA upgrade path.