SquidgyRedBall lots of great advice here.
I agree with maninawomansworld - "...do NOT let other people's opinions or desires affect your decision making regarding when YOU are ready for children. However, in terms of basic biology I'm with your mum on this one. And maryz and all who say 'crack on'!
Does your mum have a reason to say this, did she struggle to conceive? These things (fertility issues) can run in families.
My mum had trouble conceiving and had me at 33 and my sister at 35. She lost another baby about 9 years after I was born, so she would have been 42. I later went on to have fertility issues.
It may be that people who tell you you have all the time in the world may not have experienced the pain of fertility issues.
I knew fertility issues ran in my family but did not meet my dh until I was 33. We dated for 2.5 years and were engaged for 6 months. We decided to wait a year before trying for kids. So I was 37 when I started trying. We tried for 2 years and I had dd aged 39 with assisted conception (IUI).
We tried for years to have second and spent a lot of money on fertility treatment. After almost 7 years we stopped and went down the adoption route. My life has turned out brilliantly and my adopted ds is a treasure, as is m birth dd, so I am not bitter or sad at all.
However, I would advise (from personal experience) that anyone who wants to have a baby after their early thirties should just get on and do it unless there are things standing in their way.
It sounds to me, sorry, that you have not really worked out what your priorities are. Do you need a very expensive house, and if you have a very expensive house will you and your dh both need to work full time, and who will look after these children?
I worked part-time (and still do) and in the early days I found it quite hard to go off to work leave my baby at nursery. I did it and now they are both at school it is easier. By all means pursue a career and reach whatever highs you can, both of you, but if you are just working and away from your baby all day from early on, because you have no other way to pay the mortgage, you may find that parenting is quite hard. I am not being sexist, I know dads who stay at home and look after their kids. But from what you say it sounds like this new house will tie yo both up financially and make it much harder for you.
In case you wonder what I am basing this on, I am basing this on....
"Even if I worked full time we would have no spare money for nursery fees, I could pay my sister (who is a SAHM) but that would be tight." (Unless your sister is a registered child minder it might not be legal to pay a family member to look after your child.)