The 50 acre requirement is something that has been actually been revised a few times, when they were originally making the criteria they were thinking upwards of 60-70.
The location is something that is constantly debated about on KWs reddit pages every single time there is a news article about it. There's been some interesting ideas that have been floated there but they tend to have substantial issues with them. I'll summarize what's been said below.
By far the most common idea is using the existing Kitchener Auditorium lands (Ottawa and the expressway), it's just over 800m from Borden Station on the LRT, there is easy access from the expressway, it's a relatively central location for most people. The only issues with the site are that it's below 50 acres, it's in the mid 40's, and the Aud would have to relocate.
Using the Rockway golf course has been another idea floated, it's also got easy access to the expressway, it's relatively close to the LRT (Mill Station), this time the main issue is half the property is within the floodplain as Schneiders creek runs right through it.
Using the land in Hidden Valley was brought up since it's close to the phase 2 ION route, easy access to hwy 8, however again the floodplain issue comes up but this time it's also home to the Jefferson Salamander which is endangered so building anything there is going to be impossible, the area is also one of the richest in KW and they have some very loud NIMBY voices.
The Northwest campus of UWaterloo has also been brought up before since it's large enough, it's primary problems are its far from the expressway and the LRT, UW doesn't really have medical programs (pharmacy/optometry/kin/health sci aside) so its certainly conceivable that they might want to have a teaching hospital.
The general consensus from reddit is that it's going to be the Aud, North Campus or out in the middle of nowhere.
In terms of the actually properties it's been an extremely secretive process but now that it's into the final stages things have started to trickle out (how accurate is up for future discussion) but from what's been said the three properties are the Aud lands, lands out by the Boardwalk (kilometers of strip malls and roundabouts), then the last location is on land UW owns and they're potentially considering a land grant for it.