The case... is just a box with rails stuck to it. It's no more complicated than building your own 19" studio rack. Where I would NOT build my own is anything to do with the power supply. You can buy complete solutions for power. It really boils down to how much time you want to invest in building your own versus just paying for something prefabricated.
The Intellijel 1U standard is becoming more and more popular and there are other vendors beside Intellijel competing for that space now. Having 1U units is great but it's not a necessity. I like mine. But I wouldn't say that it makes that much of a difference unless you have a specific need... like air travel or limited space. Almost everything you see in 1U has a comparable 3U module. You won't be missing out if you don't go with 1U.
Finally, the resale value on Eurorack is pretty high. So there's not a huge penalty for changing your mind on a few modules. Buy or build your first case. Make it as large as you can reasonably afford in cost and space. Since you already have two semi-modular synths, start slowly. Some sequencers, a couple of effects units, and some mixer/utility modules will add a lot of possibilities to what you already have. Then you can always add more modules as you develop your own tastes and style.