My first was, and is, high needs. She's slightly mellowing with age and turning her passions towards things she's interested in. She was also dx with sensory issues but has outgrown that dx.
We have a second child that was born when dd1 was 3. She is very mellow by nature, easy, a good sleeper. When we go somewhere in public it is my 5yo who usually has behavior problems and my 2yo is fine. But as it turns out, the 2yo was born with some pretty bad health problems and that has taken a huge amt of my time and energy and was very stressful when she was at her worst. Oh, and I was on bedrest with her from 23 weeks to 35 weeks.

: While taking care of my high needs toddler.
You just never know what can happen. Probably you will end up with a mellow, healthy baby. I would just suggest that you set up a good support network before you get pg. You will want to rest alot in first tri and won't be able to because your older one will still expect to be cared for. I wish we had been able to afford daycare for my dd1. Or had family in town to watch her.
Good luck!!