Hi Susan,
Suspect you may already have information overload!
However, I went for a 250Gb ssd and it works like this. First, I don't take a huge number of images, and of the ones I do take (RAW only) I select only a relative few as being worth significant pp. My out of camera images go straight onto the regular hdd's. I review them there and COPY the ones I'm most interested into my Lightroom database which is on the ssd. That also gives me a first level of back up.
So, currently my ssd contains my software and Lightroom data base, and is less than half full. Everything works very slickly. I also have n external hdd where I back up my Lightroom catalogue and database. Of course if I want to work on a few more of my ooc images I just copy them across and import them.
I think that by the time my current ssd is full, they will have dropped in price enough that I'll just add another, and move my Lightroom files there.
I realise of course there are many CiC members for whom that workflow wouldn't do at all (and maybe one or two reading this saying What?

), but I mention it because it works for me and may for others with similar needs.
Dave