Hey Stiggy,
I spent “forever” trying to work out what size frame to get. Luckily I had “forever” time available as that’s how long it took to get one ;-)
I’m not sure your Crank to top of seat tube 450mm measurement is correct/comparable to the spec charts you read.
I couldn’t find your specific bike but found this “vintage” Trek specs doc (love the word vintage! You have a classic ?):
It shows the liquid 20, liquid 30 and the liquid 55 as having a 400m seatpost length which is exactly what the medium Stilus is.
Sounds like the your mate upgraded the stock 40mm stem to 140mm which is a big difference, but of course you can make that cheap upgrade to the stem yourself to give you extra room in the Stilus cockpit, although I wouldn’t personally ride with a stem that long due to the handling.
My advice would be your leg length is important here and your existing bike seat post and the Stilus medium seatpost are 400m, so I would suggest you go medium.
You can always get a longer stem (checkout my Stilus review on here in the reviews section, I posted a link and photos for a cheap replacement stem).
A guy on here was 5’9” on a medium and he felt like that was the max height you would want on a frame and that was with the stock 50mm stem I believe.
The other reason I would go medium Stiggy, is from my experience and this thread, medium is the least in demand frame size, so use your size to your advantage and bag a medium ???