-
25mm tyre is tight. You won’t be able to get much wider on. I can’t remember what width my rims are but they’re quite narrow lightbicycle carbon rims.
Internal cabling is a doddle, I run mine with mechanical Campagnolo Super Record 11 speed. Rear brake runs through the top tube, and the derailleur cables run internally down the downtube in plastic housing and then come out just before the BB. ISP is great - you’ll know your saddle will never not be straight. I think Look don’t advise using more than 30mm of spacers - I never cut mine down as the person I bought it off was close enough to me in size that I could make it work with spacers. If you want a pdf copy of the 595 manual I have one. I think if you need to cut the ISP down Look made a guide for the job, I don’t know how easily they are to track down, but I’m sure people on this parish can help.
I love my 595, I’d thoroughly recommend getting one; they do come up on eBay every now and then
-
Will either fit a 25mm tyre on a modern wide rim (say, GP5000S on a 20mm ID rim?)
Depends what sort of "25mm"
I've got 700×28 with 17mm internal rim on my 585, but they're really only 26.5 and that's about the limit if you want any debris clearance at all. A 25C GP5000 on a 20mm rim is going to come up wider but probably no higher, so you should be OK, since the height is more of an issue than the width.
Can someone school me on the Look 585/595? I'm thinking about tracking down one of these and getting it custom painted for a 2023 project - they've always struck me as a wonderful example of the old style mixed with modern materials and (apparently) a great ride feel.