Congratulations on the wise purchase!
Klaus C had some great points about maintenance. Just jump right in, even changing your own oil/filter and air filters etc. can save you cash and give you a better understanding of the car. Not to mention the sense of empowerment! I bought my first "brick" about 8 years ago, 23 years old, just married and pretty mechanically green! I went from being a little unsure of how to change oil and spark plugs to replacing wiring harnesses, complete rear axles and drive shafts and of course the brakes. My biggest resource has always been this website! The folks who use it are great references and always willing to share some knowledge. Never hesitate to ask questions or peruse previous posts for answers. I still have the Volvo though I've owned others and recently bought a V70, my first Volvo holds a special place in my heart. I should sell it but that will be tough :(
As for the radio; try calling or visiting your local audio place and ask them about putting in an aftermarket deck. Most of the time they can install a trim plate to fill in the gaps where the old stereo was and you should be able to get something new and installed for $200.

Cheers,
Mike