It depends a lot on what you want to see.
The first scope I used on the job in real life was a Tektronix 465 (100 MHz cathode ray oscilloscope). It was good at showing repetitive events, and that *something* was happening on less repetitive events, but was useless for one time events, or decoding bit streams, or whatever. This is, however, the general kind of technology that a design engineer or technician might have used on your vintage stuff back in the day. Historical note: the phrase "Cavemen used oscilloscopes to debug fire" was referring to the Tek 465, or maybe the 454, and comparing them to primitive logic analyzers. My Tek 475A (same thing but 250 MHz bandwidth) is generally still my scope of choice for digging into switching power supplies, etc.
I have a now-old Tektronix TDS380 (2 giga-sample/sec 8-bit digital storage scope with a 400 MHz front end). Nice for capturing edge-related problems, but memory is too limited for decoding bit-streams, etc. I keep a similar 50 MHz TDS310 in my office at work, just for general-purpose troubleshooting and to look at 1-Wire signals.
Some of the newer Rigol units are well spoken of for workhorse use. Bandwidth of 50 MHz and up, possibly with serious memory depth.
Bandwidth: a 100 MHz scope allows you to see a 100 MHz sine wave. You might need a 500 MHz to 1 GHz scope to see a 100 MHz square wave accurately.
Channels: 2 is the absolute minimum. Four can be very useful, to the point that many people who get used to a four-channel scope will not go back to 2.
A logic analyzer or protocol analyzer might be better for showing you some things.
