645 doesn't particularly appeal to me as a format, except as a compromise for someone who can't decide between 35mm and medium format, and only wants to have one system.

6x9 has the same frame proportions as 35mm, so if you're comfortable with that, and your enlarger can handle it, go for it. The Fuji rangefinders are good cameras, if a rangefinder camera appeals to you.

6x7 has approximately the same proportion as 4x5 or 8x10, so if you want to try to coordinate your frame size with the commonly available paper sizes, that is an attraction. If you're interested in the Fuji rangefinders, they also make 6x7 versions. There's lots of RB67 available cheap these days, and it's an easy system to rent equipment for.

If you like to print square or having a camera that is always oriented the same way, and I do for some subjects, it's also worth considering 6x6.

I have a 6x6 SLR, a 6x6 folder, and I sometimes use a 6x7 rollfilm back on my 4x5, either with groundglass or rangefinder focusing.