Rega rp8 vs Project extension 10

Not sure what other components you have but if equally old, you could look at saving a little on the rp8 with a rp6/ Clearaudio Concept and getting better amplification. While I am a spend as much on the source man, some older setups have awful old receivers and flabby speakers.

I really like my RP6 setup (see system link). The at-33ptg/ll, groovetracer ref sub platter and upgraded RCA connectors with furutech 126g made some very nice changes.

Well for components I have a cary slp-98p preamp with phono for mm cartridges. And for a power amp I have a cary cad120s with kt-120tubes and totem forest speakers. Which is an a great system with my old jvc and drives the speakers with ease.
So I demoed both with an ortofon black cartridge mm and both with a all tube integrated amp. But was planing on getting ether one the ortofon black to work with my existing preamp. Which people tell me its a great cartridge. I also have a large majority of old used vinyl too to use with a new turntable to keep in mind.
I also used a good selection to demo, my 3 albums Stevie ray "The Sky is crying", Daft Punk "Random Access " and Little Feat "Waiting for Columbus Live" Reissue.

I use a SLP-98P F-1, though I'm currently running my TTs through a Zesto phono stage.

The Cary's phono stage should work great with the Ortofon Black. I also had very good results running a Denon DL-103r through a Bob's Devices SUT into the MM section of the Cary. I felt that the Cary's MM phono section was comparable to that in the MiniMax, perhaps with a bit less richness in the mids.

I think you should be fine with your cartridge choice and either of the TTs.

I was considering a Pro-Ject Extension 10 but the "wall wart" as a power supply and the tonearm was(is?) very sensitive to microphonics were deal breakers. I ended up buying a 1 owner Rega P9/RB1000 and currently very satisfied. With the Rega RP8 you have the option of several cartridge upgrades, a $2500 cartridge is not out of place on a RP8. You will need to work on vibration control on any Rega deck. Many recommend the Rega wall mount but it wasn't possible for my current set up. There is no real consensus on what replacement feet work best, i'm using Deflex Foculpods($30/4 pods)and they have eliminated any edginess in the treble. Spiral Groove Strange Attractors might be the best option $450/3 feet but I have not taken the bait. Hope this helps.