HI Lara. Sorry you're going through this. I could have written your post two year's ago. We're more fortunate (if that's really the right term?) in that DH has RRMS, but we are both a fair bit younger, and at the time I felt so much sadness and rage that our futures had been taken away from us.
The forums are scary scary places - pretty negative at times, and I've not been near them for a while. They help some people, but I couldn't cope with the worst case scenario part of it all.
In terms of priorities, with PPMS get talking to them about treatment, and what drugs are available to help with symptoms and to reduce the severity/frequency of attacks.
Decide who you are going to tell and what. We have only told DH's parents so far. I'm not crazy about keeping things from my mum, but it is his choice, and I support him in not wanting this to change how people see him. This may have to change if ever his symptoms worsen, but we'll cross that bridge if it comes to it. The MS trust has some good advice on this. As far as I know he's under no obligation to tell work.
And (I know it's easier said than done) do try not to focus solely on this. I've had to make a real conscious effort to not pick up on every twinge/ache/cold DH has had since diagnosis.
I completely understand the panic - the uncertainty is absolutely the worst part of it at times. We are both 'planners' by nature and not knowing what the future holds has been a real struggle to come to terms with and has affected how we think about lots of things, but particularly our work and also not putting things off which we know we want to do.
I'll make sure i pop into the forum more often, in case you've any questions, or if you want to, please DM me.