Some recommended scopes:
http://www.rigolna.com/products/digital-oscilloscopes/ds1000Z/ds1054z/ (4-channels, 70MHz)
http://www.rigolna.com/products/digital-oscilloscopes/ds2000a/ds2072a/ (can be software unlocked for 300MHz operation)
If you want to decode I2C and SPI, get a logic analyzer:
Go type in “usb logic analyzer” on ebay as well.
You will not be very effective debugging noise with any scope that has less than 300MHz BW with good 300MHz or better probes. You also need a lot more equipment to do your own EMI prescreening. I wouldn’t trust any of those SDRs to tell you anything useful, other than maybe you have a glaring problem and now you can kind of see it with the SDR. Even a spectrum analyzer is not going to be super helpful without a LISN, antennas and some kind of a screen room.
If you are super tight on cash, best way to get the job done is make friends with people that have this kind of equipment