HI L.,
My kids are now 14, 16 and 23. When my 16 year old was born, my oldest was 7 1/2. The hardest part for me was that my oldest was not my husbands. Her brother and sister were always the apple of their daddy's eye, while my oldests dad did not want anything to do with her. Things are different now, as my oldest has two boys of her own, and they are the apples of their grandpa's eye (my husbands).
My brother is 12 years older, my late sister was 14 years older, and my younger brother is 8 years younger than I. We all had different dads, which made it difficult to be raised together,as my mom did not have custody of my older siblings. Also, there was a generation gap of 52 years between my brother when he was born and his dad (my step-dad).
I don't think there will be any problems in your family, because your children have the same dad. Just pour love on them all equally.
As for baby items, there are a number of baby shops around, that provide gently used or brand new items, there is no need these day to purchase brand new items. Plus if you are blessed to have a baby shower, you will surely get some neat things.
Sorry to hear you have not had a great start to your pregnancy. One thing thaat helped me was to have soda crackers next to the bed, so that before I got out of bed, I could eat a couple of crackers to ease the nausea. As for not gaining weight, don't worry it will come! Have you started your prenatal vitamins? You can even add ensure to your diet, as it will help you keep up your energy.
Have a blessed pregnancy, and take care!
C.