I actually like having both "real" "skinny" ball locks and converted pins as you only usually really only need one or two of the skinnies for all of them to fit in a keezer.. what I mean by that is, for example my 7.1 cu ft luckily can just make 4 pin locks, but if I put one or two ball locks in there then it's a loose fit. And guess what is better to put on the compressor hump.
I have several of the converted pins for about $45/ea shipped from on-line sources.. in the last two years or so... The balls were all in the $55 category shipped... wait for deals and sales.. free shipping goes a LONG way with these...
My favorite ones are the beat up ugly single handle Cornelius because 40 year old plastic handles don't need to be soaked over and over again in silicon spray to stop getting black on everything like the rubber handled ones are.
As for build quality? The Cornelius branded ones are the flimsiest... the Firestone ones are the toughest. I used to think the Pins were just more substantial than the Ball locks and then I lucked into a couple of local used Firestone Challenger ball locks.. it's the manufacturer, not the type.. literally the SS is a gauge thicker.
As for converting pin to ball there are two standard types, most are the "Firestone" 9/16-18 posts. Any "Cornelius" pin-lock will be 19/32-18 (new AEB are supposedly that too).. I've never seen any other type, but the "racetrack" type which are really old are different.
Most places sell the conversion kits for $10-ish, which is a good idea anyway as you want to do all the o-rings AND poppets when restoring a keg anyway, and a couple of universal poppets runs about $7 anyway.
As for going to Sanke? Yeah, doable.. that does have a whole another issue for sanitization.. once you get it down it is likely better but I like to see the inside of my kegs when cleaning, personally.... so the simplicity and appropriate batch size of Corneys are why they became the standard.
I suspect that the $70 ish sale price for new will be the upper bounds of the used market when all settles out. Expensive, yes, but likely you can get your money back out of a keg when you decided some day to sell it.