+1 on the above.

I have two 2003 Poprads (actually one Poprad and a Wayzata that I converted to drop bars...which is a Poprad).

38mm tires will fit fine. I run Panaracer Gravel King Slicks on one bike and it's a great tire. Fast and nice ride at around 60-65psi. The other one I run Schwalbe Marathon Supremes on in size "28x1.40"...aka size 37-622 (37mm wide, 700c rims). The Marathon Supremes are a wonderful tire. Super fast and cushy ride for roads..not great on gravel, but will do rail trails fine if they aren't wet/greasy. When the Gravelkings wear out, they'll be replaced with the Marathon Supremes.

You might be able to get a 40mm tire on a Poprad, I'm going to try when the GravelKings wear out. Certainly a 40mm tire will fit on the front, while fit on the rear might be iffy at the chain stays.

With respect to the fork..I wouldn't bother. With good tires the stock fork is fine.

Even if you could add disc brakes, they'll mostly just add weight. If you feel you need better braking..pick up a set of Tektro 720s and mount Koolstop salmon pads on them. You'll stop..seriously..you'll stop. If you set them up right and take the bike out for a spin..and you're not use to them, you can launch yourself over the bars if you hit them too hard. (From experience when a friend took one of mine out for a spin..he was used to canti brakes that were not set up properly and squeezed the brake levers a bit too hard. Fortunately he stayed upright.)

I have a good number of bikes, but the Poprads easily see the most miles.

oh..and I run both of them with Brooks Imperial saddles. Not sure what you're looking for in "comfort"...there's different approaches to comfort depending on what is uncomfortable.