I believe you will experience better performance in Lightroom if you have your images on the main hard drive. Here is a link to a page of things you should consider when moving Lightroom to a new computer:
Since you have your images on an external hard drive, it will be necessary for you to reestablish the link with those images on your main hard drive. Here is a link that might help you in that regard:
The transition to a new computer can be relatively painless if done correctly. I moved to a new computer about a year and a half ago, and after I reestablished the link with my photos on a second internal hard drive, I was right back in business using Lightroom. Good luck on the transition.
The license that you have now will enable you to install the same version of Lightroom on the Mac. You will not have to purchase a new license.
Regarding the issue of putting images on a fast drive or a slower drive ... it makes very little difference, you probably wouldn't even notice. See this study by Ian Lyons (which despite the title, includes comparisons of using an EHD to using an internal HD): Will an SSD Improve Adobe Lightroom Performance? | Computer Darkroom
You would want to put your catalog file on the fastest drive you have.
Thanks for your helpful advice. I'm sure it will all go smoothly when I pluck up the courage to do it. I've had issues before with broken links when I first started using Lightroom so my experience then will stand me in good stead
Moving from my Windows machine to my new Mac was pretty painless for LR. I don't have much in the way of special configuration for LR, so I just copied the database over and when LR launched it looked to find the file structure for my photos. I didn't copy the previews over and just rebuilt them, which took a while.
As for your setup, the more backups, the better. Since Time Machine allows for backups to multiple drives and if you've got drive space to spare, I'd copy everything onto the local drive and have multiple backups (for me, I have two time machine backups (one local, one offsite) and Crashplan for my photos + database).