Borrow as much as possible as long as you dont mind if it doesnt match. Especially the stuff they will only use for a short period of time - like moses baskets, and swing chairs. Things worth buying that you will use alot and/or for a long time, are cots, highchairs, carseats(altho you could probably borrow the ones they use in the beginning - you know the ones with handles that face backwards) and invest in the next stage up which thye get to surprisingly quickly. I would recommend putting hte babes in the same cot to begin with and then buy a second when they get too big. Dont buy a changing table as you cant keep a hand on one, while the other disappears out the door. On playpens, just get good gates that go across doors, and then will go across your stairs, best way to isolate them, and get dual use out of the equipment. Travel cots - definately two, I am still using mine ! (but one was second hand from a friend who'd finished having babies). Lots of bottles and bottle warmers, and a big pot to sterilise in are good. Also industrial quantities of things you dont want to borrow (or for that matter pass on when you are done) - like bibs, muslins, vests, basic babygros, nighties. Definately a double buggy, but you might also use Baby Bjorns for the first little while if you have dad around. A few changes of sheets are useful too. If anyone offers you a present - a couple of months worth of Pampers is a good one, altho bear in mind tha tthey grow - so dont stock up on a size that they wont wear for ever ! And formula is expensive so buy in bulk once they are settled on one kind that they like and tolerate well. Someone also gave me a big glass pitcher with a rubber lid for keeping boiled water in the fridge in - wonderful present as one is always making up bottles and not wanting to wait for water to boil then cool.
I am rambling - but to sum up the things I am still using are the travel cots, my cots, the double buggy, stairgates,and used my basic IKEa highchairs for ages. Best things I borrowed were moses baskets, swing/bouncy chairs, excersaucers, those bouncy things that attach to the door frame, a nappy disposal unit, baby baths. Good luck to her. Its fab.