Welcome to the forum!
I can say up front I'm not going to be much help with regard to PixInsight, but I just ordered a 27-inc iMac which is pretty much maxed out, except I went the the 512 GB flash, if that's what you mean by 'SSD.' I have 512GB flash on my MacBook and wherever possible I am done with spinners ..
I know this is sacrilege
![laughing :D](http://pixinsight.com/forum/Smileys/default/laughing.gif)
but I do use programs other than PI; I went with the the 4GB video card since I think it will help with my other photo editing, like, in Lightroom, maybe even 4K video rendering. I don't know that for sure, although I do back that up with an unsubstantiated intuition that the better the specs the longer the machine will have acceptable performance.
I also collect other overly-expensive things like telescopes and cameras so I'd be wary of taking any of my advice
![smile :)](http://pixinsight.com/forum/Smileys/default/smile.gif)