The numbers: 45, 60, etc refer to the diameter of the blades. Just a uniform sizing thing.
As for which rotary cutter? Try different ones. Which best fits you is important. How do you hold it, do you have any disabilities, etc. I started with an Olfa but went to a Fiskars then back to the newer Olfa due to personal hand limitations. Things change over time.
I hope you are able to join a guild or group of quilters as we love to share. Heck, I got my stash started by guild and group members. I'm ready to start a newbie now.
If budget is a concern, then there are ways to keep your costs down til you are ready for more.