I can only say what I did; which was to get a 125, in my case a Suzuki RGF 125, which I rode for a year on L plates. The DAS schemes had been brought in just a bit before I started and the rule about having to pass on a 600 if you wanted to ride unrestricted straight off. So, to do this I booked a 3 day course culminating on the day of my test which I passed.
The main thing that sticks in my mind is that the instructor made a comment after an initial ride that I had ridden before, the other guys on the course hadn’t, obviously down to my previous year. I know things have changed, but back then (and now) I wouldn’t contemplate on going from nothing to full licence in 5 days or whatever.
So my advive would be to ride on L plates for a decent amount of time.
As to first bike, I took the view that most could kill me no matter the power, but was more likely to be another driver. So my first big bike was Suzuki GSXR 600 :-) happy days
Oh, and yes, Do IT