Seeing as Sony seems pretty eager to put out HD collections of classic series (Sly, Prince Of Persia, etc.) it doesn't seem like a Silent Hill collection would be completely unthinkable. As someone who's never played any of the games I'd love for it to happen.  
 
Do you think it's reasonable? I mean, the first one is graphically pretty dated (and I assume that's why the Tomb Raider HD collection did not include the PS1 games), but the other ones shouldn't be too hard to fix up and polish. 

I don't know. I've seen SH2 and SH3 on PC and honestly the higher resolution did not really do these games any favors. I am a ridiculously huge Silent Hill fan, but I would not really put them on the list of games that would benefit greatly or call for some sort of current-gen rerelease. I'd also kinda hate trophy pop-ups in them, as 2 and 3 make sure not to disturb the game world atmosphere with HUDs and OSDs.
 
Just play them as they are right now.

I don't think it's very likely.