Ok thanks that is very helpfull to know
How high does my memory/ sample rate need to be to see I2C and SPI at work?
It depends what you mean by "see". Some scopes provide decoding in hardware for these.
The more memory you have the more bits that you can capture and then zoom in on.
At the lowest level I'd say you want to capture data at a rate that gives you 10 points per clock tick (so you can see the shape as well as the level).
At say I2C at 100 kbits a second this would be only 1MS/sec which is very low. Most scopes should be able to go at 100 times this at least.
SPI according to Wikipedia typically talks to peripherals at up to 100MHz. To accurately observe a 100MHz clock and get accurate rise times and so on
might require a very expensive 1GHz bandwidth scope.
For hobby use you're probably going to be working at much lower clock rates.
If you want to look at the shape of clocks that are around 30MHz then you probably want a bandwidth that doesn't attenuate much up to the 5th
harmonic so a 200MHz bandwidth scope. The Nyquist sampling rate needed would be at least twice this or 400MS/s but in practice you really
need to have 1GS/s
More advanced scopes add features like special triggers so you can trigger off particular bit patterns and also decode the data on screen.
The main requirement you may have for SPI is the number of channels on the scope. If you want to look at MISO MOSI and SCLK then you need a 4 channel scope
rather than a 2 channel scope.
I think though that a 4 channel scope with 100MHz bandwidth would let you check the logic levels and timings in most circumstances. The memory question is
difficult to answer because it depends how many bits you want to capture at a time.