1. You could say I prefer custom...but I am biased.
2. It isn't as easy as i think you are currently thinking it is, but it isn't rocket science either.
3. don't rush it, read up on it and know where to look for guidance.
4. They won't build up that light. Almost looks like you didn't add the weight of the spokes or nipples. Regardless .....sub 1300k tubulars built using non-branded rims from ebay....have fun with those. I have had my hands on a few that people have brought me and ......yeah.
5. There are a lot of options out there that will allow you to get into a durable carbon wheelset for a price that is less than you would figure. Not trying to shill myself here....there are others as well. In these cases you don't have to guess about what will or won't work and hope for the best.
6. If you want to build your own wheelset - start with a set that is easier tobuild - has less risk. Go with proven builds and known components. Build it and see what you learn. Ride it and learn how what you do impacts how you ride. Then wen you are happy go sell that set and build a new one. Next thing you know you might become a custom wheel builder.
7. Enjoy - and learn that no one else can spec a wheel for you better than you can spec a wheel for yourself - except maybe a builder who rides with you and knows you well.