What's your budget and what will you be using it for? I was considering a SSD for my new build, but decided against it. You could set up two HDDs in RAID0 and get almost the same kind of speeds with much more space for more or less the same amount (maybe even cheaper depending how much space you want). Installing a SSD later (when they are cheaper) is a good idea and would be relatively painless. As for the PSU it's hard to say but if you are going for a mid-range gaming rig I think 500W is recommended.
iMac Pro review