Do SSDs improve speed of processing applications eg loading or running Office apps or is the primary reason for faster boot up/shut down? Thanks!
Yes they do and imo no it isn't.
In my opinion there's too much misplaced importance on bootup and shutdown speeds. ALL operating systems take time to boot up (and shut down) for a reason - they carry out important checks to the computer's hardware and sofdtware that (imo) are vital to the machine's smooth and stable running.
If having to wait an extra minute or so for a machine to boot up means it's self-tested then that's fine by me. Having said that, SSD's do contribute to faster start and stop times, but not by a great deal.
If you're buying a laptop then it's unlikely you'll be able to use two hard disks which is a pity as (imo) the best system is a combination of SSD for operating system and a large mechanical disk for storage.
A 1Tb SSD will make a laptop expensive but if you have the budget then go for it - it will give the best performance. However, for economical reasons I'd likely go for a plain mechanical drive or for a little extra processing speed - a hybrid drive.
If using a mechanical or hybrid drive I can't stress enough the importance to BACKUP, either by burning to DVD's (realistically 4.2Gb a disk) or Bluray disks (realistically 22.5Gb a disk) or to an external hard disk.
So, if it were me I'd go for i7/8Gb RAM/1Tb Hybrid drive/Bluray optical drive capable of burning Bluray blank disks. Unless you're into the latest computer games or some heavy duty CAD or video processing then 8Gb RAM is plenty.
Good luck.