Designing New HVAC System

I recently purchased a 90 year old, 1500 sf house that I plan on entirely renovating, with as much DIY as possible. It has a 40 year old HVAC system and what look like original ducts. Central air was added with this system, but there is no return duct on the second floor. Also, most of the supplies are positioned towards the center of the house, so the circulation is not very good. I would like to replace the furnace, AC, and all of the ducts. I was an HVAC technician for about 5 years and did service and many installations (residential). I'm very confident I can install all of this stuff, but I don't have any experience at designing it. I can probably figure out the manual J easy enough, and possibly the sizing of the ducts, but knowing how to route all of the them is a different story.

I did find this website (https://www.ductworks.net/hvac-layout-design-services/). They say they can design the entire system. Does anyone have any experience using a service such as this? Any other ideas of how to approach a project of this magnitude? I plan on eventually gutting everything and insulating, rewiring, etc., so opening walls and such is not a problem. I also have really no time constraints so I can take my time.

 

I have no knowledge of that company. IMO if I was gutting a 90 year old house or any house for that matter id put in a few ductless split systems. No leaking duct, easy to install. energy efficient redundancy.