I brought or accepted all second hand my Dt's and never regretted, children get more expensive, only in the last year have I had to buy new as boys apear to be very tough on clothes and people buy less for boys than girls, I was lucky another mum at nursery remembered me when moving and gave me all her Tb's clothes up to age 6 including underwear, which she was sweet about, packed separately and told me if I found it weird then I could just chuck away, I didn't.
Like the others I spent an indecent amount on mattresses and car seats, I felt totally justified in these purchases, I got 2 isofix car seats £200 each and the fixing £150 each, so new an idea at the time, I had to show the salesman the blurb in the car manufacturers web site, they where brilliant, just clunk in and press a button to release, a real bonus with 2, couldn't give them away after, wish i kept them as 2 cousins have had twins in the last 2 years and would have made their lives so much easier... to be far never seen anybody else with them in the intervening 8 years, possible the cost was too much, they are much safer than standard car seats and yet a bugaboo...something you can show off is a "investment" item.
Oh! I also brought new, a slightly rusted on the poppers pram/pushchair from a real old fashioned bike shop it was their display model for £160 pounds, found out if was some fancy Italian make and my aunt sold it for about £400 on e-bay.
so your mum. she needs to buy something lovely for her grandchild, money in a bank does not do it for me to be honest, my Mum in law knitted some lovely things which i have kept as mementos after use, in fact everything people made me I kept.
one thing you might not have thought of is washable nappies, the posh type, about £300 the set but can be used for the next child (no pressure from me then :) ) not for everyone but I was having to do bi-daily shopping trips to get disposables (the logistics of twins is not easy) and the nappies where fantastic, I kid you not, loved them.
high chair, you might not have got this yet, as not needed for a while.
if you haven't brought a Mose's basket or crib as they are used for literally weeks, she could buy new, they are adorable. I brought second hand (one of each as that was what was available second hand) and they are so handy for those weeks, as you can set it up downstairs. (to be fair one of my twins was so small he was in the crib until 7 months old LOL! )
Tumble dryer if you don't have one, I didn't until they boys where born.
Baby prison (playpen) I got a hexagonal one, which opened out to fix to walls Very handy for when the phone rings, doorbell goes even when tiny babies if mobile and that could have a tent like a castle I think (I never brought that bit) again I doubt you have thought that far ahead.
good luck and congrats,