What will matter is not the maximum sample rate for the scope, but the fastest sample rate the scope can use while capturing a long enough time window to be useful; in many cases, that will be limited by the length of the scope's data buffer. If you need to capture two consecutive SPI transactions which are separated by some amount of time, you may have to "zoom out" pretty far enough that the scope's sample rate drops to the low MHz. Depending upon the scope's buffer size and the amount of time between transactions, even that may not be enough.
Deciphering SPI data may be possible if the scope's sampling interval is no longer than the amount of time that the SPI wires are "stable" between transactions, but the data will be much easier to read if the sample rate is at least twice that, so that the lines will be stable for at least two sampling intervals. For a 500.0khz data rate, a 2MHz capture rate would probably work decently; one may occasionally end up capturing things right on the transition, yielding an "ugly" trace, but even then the data should be decipherable even if it's a little hard to read.
Note that if you're interested in trying to determine why an SPI receiver isn't getting the data that was sent, you may need to use a much faster sample rate (so you can judge whether clock-signal edges look clean, setup/hold times look decent, etc.), but in those cases you probably wouldn't need to capture data for as long, and you could thus use a faster sample rate. For that purpose, at a data rate of 500Khz, one would be hard-pressed to find any non-"toy" scope which wouldn't be adequate.