Marc,
The only real differences between the 205 and 203 cameras are in the metering characteristics, the 203's meter metering center weighed, the 205 doing spot.
And the 205 has an additional zone metering mode, in which it displays how the metered points are distributed on a contrast/zone scale.
This works in conjunction with the extra dial on TCC-magazines, which lets you set the Zone System's n-value: a measure for how much contrast will be changed during processing, such that a value of -1 lowers contrast (i.e. compresses the contrast/dynamic range) by one stop, +1 increases contrast by one stop.
Overexposure (changing the ISO value) and shortened development lowers contrast, underexposure and prolonged development boosts contrast. By how much development times must be shortened or lengthened is something you have to find out for each B&W film you like to use, and fine tune yourself.
The 205, in Zone mode, will read the TCC-magazine's n-dial and bias both ISO and contrast values, so that the measured points are displayed correctly on the contrast scale.
Once a particular n-setting is used, the entire film is committed to that n-value. So you will need about three magazines to cover everything (one for normal (n=0) shooting, one for a particular low or high contrast situation you might encounter and need to adjust to, and one in reserve, in case you happen upon a scene needing the opposite (or more of the same) of what is already set on your other backs).
In short, if you do not use the Zone System, you do not need TCC-magazines - but if you do, get three.
Nor, some would say, would you then need a 205.
On the other hand, it's nice to have the option, even though you'll never need, or even use it. That's the definition of luxury, isn't it?
Besides, your kit doesn't fit in your bag already, so why not?