I only really have experience of the satnav stuff.
If you just want a radio, there's some very cheap Chinese stuff available. Not sure of the reliability, but there isn't a huge outcry about it, and some of it claims to have a built-in learning of the steering wheel controls. But I've not seen one in action.
If you go for the main players - Pioneer, Alpine, Clarion, Kenwood, etc. You will probably need a connection harness and another for the steering wheel controls. if you want to control the radio from the steering wheel. If you don't, all the other steering wheel functions will continue unaffected, and the warning sounds will also continue.
When I did the upgrade 5 years ago, there were no harnesses for the satnav, so I hard wired it. Not too bad if you think logically and identify the many wire and work methodically. I never bothered with the steering wheel control of the radio. It's mainly the volume and seek. If you only listen to one station, it's not worth it!
If you have a boot CD changer, it won't work. Divorce it before you remove the radio and sell it.
The AS2 & AS3 systems have the amp. You may find that you are better off with volume control if you buy a head unit with RCA/phono coax line level outputs (0-2volt) which is the voltage level the amp expects. If you feed it the normal harness output from the standard speaker feeds (0-6volt) the volume gets very sensitive and can be very loud - maybe break something.
I also fitted ground loop isolators. Saab use a feed and return between the head unit and amp. Mosy aftermarket ones use live and earth. Can cause humming.
Look at sites like Connects2 & InCarTec for the connection bits. Things change all the time. I believe that InCar Tec are based in Somerset.