we have twice travelled through paris getting from eurostar to montparnasse for our onward TGV using the metro, once with a 7 week-old, once with a 14 mo (same baby!). we carried all we needed for a week's holiday (including washable nappies) for the 3 of us in two 65 litre rucksacks.
the metro is not very pushchair friendly, but we managed because the rucksacks meant we both had our hands free, so we just carried the pushchair up and down steps. when dd was awake we sometimes removed her from the pushchair and carried her on the escalators (some people think it is safe to put a pushchair with a baby in it on an escalator. i am NOT one of these people. i have no objections to empty pushchairs on escalators though). i have seen lifts at some stations, but not gare du nord. also, they are very slow, probably to discourage the able-bodied from using them.
presumably once you get to wherever you are staying travelling around will be easier cos you won't have so much stuff with you. i imagine the bugaboo will be fine as there are two of you - one can carry the baby, the other push the empty pushchair whilst on the metro.
once we were on the actual train people were very accommodating, smiling at the baby, not minding me nearly falling on top of them cos of the weight of my rucksack etc.
btw slightly irrelevant to you, but i was surprised to notice that a folded maclaren techno xt pushchair fits into the overhead storage on the eurostar - i wouldn't have thought to try it myself, but i saw someone else's up there, so that's where we put ours too. in fact you might have more trouble stowing your unfoldable buggy on the eurostar than on the metro. i don't know though, i am not experienced in the ways of the bugaboo. does seem a bit excessive and a shame to me to have to buy another buggy when you already have (a very expensive) one.
fyi it is cheaper to buy a 'carnet de dix tickets' on the metro than individual ones (or even 8 individual ones like i tried to buy).
also, if you are travelling by train to london to get the eurostar, consider getting a family railcard. you have to buy a ticket for the child, but everyone's tickets are discounted so it normally works out cheaper. also, we once managed to buy ordinary (ie relatively cheap) saver returns with our railcard on an 8am train where it would have cost over £200 without the railcard (seemingly saver returns not available when not travelling with child). you can also buy tickets from x to waterloo ie no need to pay for tube and often at a bargain price, but you need to buy in advance and have to bring your eurostar tickets to buy them - presumably to prove you are genuinely travelling.
last also - there are 'family friendly' carriages on eurostar, but i think you need to ring up to book them, you can't specify online. they are the ones right next to the baby changing rooms. but, you may not want your (possibly sleeping) child disturbed by other children.
sorry for rant, but i am evangelical about rail travel, it is definitely the way forward. what child wants to be belted into a car for hours on end, when they can be wandering (or crawling) up and down a train?!?