Jelco sa 750e

@supatrac Welcome. Introduce yourself, and let folks know if you are a manufacturer. Start a thread, etc.

I’ve looked at this before and it’s definitely interesting.
SUPATRAC Blackbird (supasound.com)

Ortofon tonearms were made by Jelco and are no longer available.

Yes. For most intents and purposes,  it doesn't matter how big the hole is. 

@jeffrey75  I couldn't budge the original stub on the older silver 750 so I rang Expressimo. Apparently some stubs can't be undone as they're glued or something. I tried it again on a newer black 750 and it was very easy. I also tried the brass weight only on the 850 but the original sounded better.

@52tiger 

 

See my Virtual System.

@jeffrey75 I have a 750L 12" with Ammonite collar and Expressimo brass stub and counterweight on a Garrard 401. Not much to say other than it’s great. It’s not as good as the 850 though. I will say that the better the turntable, the better the arm performs. In other words, the 750 and more specifically, the 850 sounds a lot better on my higher spec main 401 than on my second 401.

I run Decca London (very low compliance) on my 750/850s with no issues.

A 750 with a Ammonite mounting collar is perfect for medium to high compliance MM carts and some lighter medium compliance high output MC carts

@52tiger 
http://brooklynaudioinc.com/index.php?route=product/product&product_id=1954

Have you tried these guys?

Notwithstanding your next steps, upgrading your 750 to 850 will yield a nice improvement. They're hard to get now but worth it. Drop in replacement. 

The better the table, the better a good arm will perform. Your turntable support is also important. When I moved my table from wood to metal stands to concrete blocks, the improvement was a major upgrade. Adding springs under the table instead of cones also brought a large improvement. Try these 'upgrades' before spending relatively large amounts on gear.

Look for my thread on the 850. Basically, it's more resolving, finer etched, blacker - more contrasting, better layering, more spacious soundstage, faster, more color. It's a no-brainer. I haven't heard the 950 - uses a spring to apply downforce rather than weight alone for theoretically better tracking. 

Jelco TK-850M MK III

Mk3? 
Yes, the 850 is the logical choice if you don't want to spend too much. I do believe that bkeske is correct about Origin Live. @millercarbon might know.
Not sure what your longer term plinth plans are but going 12" will prove more satisfying in the long run.