No they are not, they only improve boot times and software load times, not 100% necessary, you can chuck one in later.
The way I see it, you can just slot in an SSD, you can't pay the difference to upgrade your GPU or CPU later, you'd need a whole new investment for each. For that reason it's best to stick with a HDD now and migrate OS to an SSD when you can afford a solid 240/250GB one.
By the way I forgot to ask, what's your monitor situation?
You'll want a 1440p or 1080p 144hz monitor with the 1080.
You can stick with 1080p 60Hz though just fine, even though it'll be a bit overkill.