My DD#1 was 2 when our twins were born, plus DS was 4, so a bit different from you, but here are my thoughts;
Help - Yes, you will need it. You might be surprised at how many people will offer help. We got a lot of support from friends at our synagogue. We had a few teenage girls that regularly came by to help out when they had free time. Some had to do a required number of hours of community help as part of their school program so we didn't have to pay them. We just bought them a nice gift once we didn't need them anymore. Anyone that offers anything - just accept their help. I was in such a daze the first few months that I was happy to give anyone that walked in the house a baby to hold.
Minivan - We have the 7 seater Sienna. We have the twins in the 2nd row captain's chairs, pushed together on one side and the older two in the third row. I have no problem leaning in to buckle up DD#1.
Stroller - I would just get the DSNG to start with. You are not really going to be able to get out of the house much with all three at the beginning. You can try looking for a Valco on Craigslist. We already had a double but it doesn't have the toddler seat option. I thought for sure I'd end up buying the ABC or the Valco but by the time I was ready to start going out with all the kids, DD#1 was fine walking on her own. I would wait and see what your needs are once the twins are here.
Monitor - We just have 1 in the nursery. We didn't need one for DD#1.
Sleeping - We just used the PNP with a bassinet that we had from before. Our twins were born at 35.5 weeks and weighed 5.5lbs each. It took about 2 months until they were too heavy to sleep in the PNP together. I did find that I needed more places to put the babies down than I did with my other 2 kids. We bought a 2nd PNP because I wanted another one with a bassinet and none of our friends that were offering to lend us theirs had the bassinet. I knew I was having a c/s and felt it would be too hard to be bending down into the main part of the PNP. We kept 1 PNP on the main floor and 1 upstairs in the bedroom.
Bouncers - That is the 1 item I felt I could never have enough of. We had 6 (most were borrowed) - 2 on each floor of the house. We only had 1 of most other items. If we saw that both babies were spending a lot of time in something we tried to borrow a 2nd. I never felt the need to have matching stuff. In fact, having 2 different styles of the same thing just makes it more interesting for the babies. Less chance of them getting bored of watching the toys on 1.
Highchair - By the time the twins are ready for solids your older DD will be 2 years old. I'd just get her a booster and get a 2nd highchair for one of the babies. I wouldn't even think about it now unless someone wants to buy you one as a gift.
Feeding - I did a combination of BF and FF. I would not get the twin pillow for only FF. I never liked the way it worked for bottles. We used bouncy chairs for giving them bottles. Or, I would hold 1 baby in my arm and cradle the other in my leg. We used rolled up receiving blankets to prop the bottles when we needed to.
Schedules - It's possible to have them on the same schedule but I wouldn't sweat it if they aren't, especially at the beginning. If 1 baby woke up and was hungry I would just try to feed both but sometimes they weren't both hungry. From about 3 months onwards our girls have been on the same schedule but before that there wasn't much of a schedule around here.
Congratulations and good luck!!!!
Twins are a lot of work at the beginning but they are so much fun!!
DS - 10
DD - 8
Twin Girls - 6