Digital oscilloscope minimal sample rate to capture SPI data going at 500 kHz
More than adequate, as I think any scope will be for sampling 500 kHz. 500 kHz is a period of 2 &micro;s, so at 250 Msps that gives you 500 samples per period, enough for measuring time with a 0.2 % accuracy.  

Note that the scope's bandwidth is enough to capture up to the 50th harmonic of a 500 kHz signal. Even with a brick wall filter, cutting of all further harmonics (which doesn't happen), as a first approximation your square wave will still look like this:  

  

So the edges of your signal won't suffer from the bandwidth limitation.