I bought my front from Bavarian autosport. 129 for meyle bearing, nut and cap. I know you can find it cheaper than that. Ireland Engineering replacment part section has meyle for 87, the skf for 150 and the nut for 7. You will also need a 36 mm socket I took my bearing and nut into autozone to find a socket. Although they had several 36mm “wheel hub sockets” none of them fit into the center of the hub and I ended up using a regular 1 1/2 socket. I suggest you order a socket made for e30 hubs when you order your bearings.
The rears I just got from IE yesterday for 51.49. I have never done the rears before but I will be rebuilding my cv joints at the same time since the boots are rotted through. I assume that the axles need to be removed for the replacement. The 6 bolts that go through the inner cv into the diff output flange love to come out. I would say clean them spotlessly and then use permanent locktight. I plan to drill and lockwire mine this time. When they come loose they will do damage including a shattered differential cover or worse. Ask me how I know this.
I don’t see a reason to replace them as preventative maintenance. I have always had bearing make noise before they have any play. Every Noisy bearing I have pulled off of one of these is perfect except for a tiny defect in the surface of the inner race. I suspect that these start out as an imperfection just under the surface like a bubble or impurity. The metal over the defect is stressed until it breaks off after a few years and you’re bearing starts to wear and make noise. If your bearings have been on the car for 20 years they were probably made perfectly and are more likely to last another 20 years than the bearing your about to put on.