If you want to start cheap, the 750x is great as well. Please double check this, but I think it has two extra inputs for another cymbal and tom, that Thoman sells as an upgrade kit for like 70€.
Regarding the kick tower, most of the cheap "just pedals", don't have the rebound you need to play double strokes or fast notes like you would on a real drum kit. So if you want your skills to be transferrable, you probably want a real pedal. The higher end Roland pedals look like they'd provide good rebound, but they're >200€, so I doubt you'd be saving any money with them. They're smaller and potentially quieter though. As a bonus, the towers work with double pedals, if you happen to be into Metal Music.
You can approximate the length of any path (including circles) by adding the lengths of many small line segments that follow that path. Making a line segment bigger by some factor, will increase it's length by the same factor. Therefore, scaling the circle by any factor, increases it's circumference by the same factor. Scaling a circle is just scaling it's radius so: Scaling the radius by some factor, changes the circumference by the same factor. That means the ratio between radius and circumference is always constant.
I hope this is decipherable :D