Depends on how accurate you want your measurements to be. Some where between 2x fmax at minimum and 5x fmax for high accuracy.
So you're looking at ~300Mhz to ~1Ghz for bandwidth of your scope.
What kind of approach to testing do you prefer amateur or professional?
For amateur level you can build some analogue test instruments very cheap.
Professionals use expensive instruments and can achieve better results.
A high bandwith oscilloscope is nice to have but not vital.
The most usefull thing you can have is good radio receivers. You can listen to your oscillator on an SSB receiver, short term frequency drift or really bad phase noise is obvious to the ear.
A simple high impedance rf proble with a voltage meter is very usefull.
You can use a wavemeter to check the output of transmitters for excessive harmonic output.
A grid dip oscillator can be usefull. Making one yourself is a good project to get started.
A signal generator and a frequency counter.
>can i use PC sound card (software) based tools.. ???
Only for audio frequency stuff.
Some of these instruments throwavay18 already mentioned. There exists some more of them like SWR meter, spectrum analyzer, sweep generator etc. I am sure you can find them on internet specially in radio amateurs pages (ham radio).