Go test riding.
I moved from a 600 Ninja (first bike) to a Z1000 naked beastie last year. I much prefer the upright naked and don't miss the sheer speed of the ninja at all.
Personally I think a 600 or thereabouts is big enough to get used to however the speed controller in your right hand so it's all up to you. There is nothing 'too fast' for you if you have self constraint. I would say get something light though as dealing with a heavy bike will be tricky until you get used to it.
The ER-6 is a decent bike, I like the naked one - similar styling to my Z1000!
I like Kwaks, good value for money in my opinion. My mate has had a CBR600F, CBR600RR and now a Fireblade. I wouldn't say the Honda is any better than the kwaks but you sure pay more for it!