i think a variable speed 4.5 or maybe a compact 5 is a good way to go for a first grinder. in my experience the 5s are slightly more powerful, beefier, and heavier. unless i needed that all the time, i'd rather go to a 7 or 9 for big cutting and grinding jobs and have the smaller 4.5 first grinder for regular stuff. in the states, 5/8x11 is a common 4.5 size and you'll also find wire cups, wheels, brushes, etc. with this and i'd want it on a 5 as well. with enough power you can also run a pad and sanding discs on either.
my suggestion would be a high quality variable speed 4.5 or 5 to start. add a 7 or 9 later for heavy duty stuff. and a cordless 4.5 lithium ion to cover the spread. having a backup 4.5 so you wouldn't have to change between cut off, grinding wheel, brushes, etc. as often is handy too.