The car should be ok but your fuel consumption will probably be at
least double of what you'd get driving 55mph.

none,  

just do your intervals @ 10.000 Km
use  highgrade  oil, and change the oil/air/feul filter
change your cambelt more often then spect
my old 305 did about 300.000 Km @ 160 Km/h ( 100 miles)
and it still runs fine.

100mph at 3500rpm?! bloody hell, my 306 1.8 petrol i'm sure needs to go to
at least 4500 to get 100mph out of it...somethings wrong here :-P

As the engines only doing 3.5K RPM, there should be no adverse engine
effects (What's the red line on this one 4.5 or 5K?)
The rolling chassis is going to see more abrasion from bumps n stuff, but I
wouldn't think massively, particularly as you're on M/ways.
The only problem I have with doing this kind of speed regularly is worrying
about the survivabilty of a front wheel blow out,  particularly as the tyres
are going to warm up.... but then I whittle a bit ;-)

Rick.

My 306 TD hits about 95 at 3500rpm, very rarely get to 100 as it runs
out of puff around there. I do about 100 mway miles a day and normally
sit at around 3200rpm (allegedly). The car has survived this ok so far,
I bought it with 20k on the clock and it's not far off hitting 100k 2.5
years later. Only faults so far are an A/C pipe leak which I haven't
bothered to fix and some miscellaneous oil leaks, which I haven't fixed
either. The main one being from the power-steering rack and it just
needs topping up every few thousand miles.
On the subject of power steering leaks - has anybody tried the "stop
leak" products from STP and Wynns? Is it just snake oil?

The car still drives like new, it could benefit from a new clutch cable
and a general valet (which it'll get at 100k) but for a motorway slogger
it's brilliant. No plans to sell it even though after an upcoming house
move my commute will change from 100 mway miles to 40 odd on country
roads.

Top tip for motorway driving - stick to Michelins, I'm still on the
original tyres at the rear :-)