That 121 is probably going to set you back the cost of a MAF, I have not seen a great deal of ghost codes for that one. If it posts it you need it.

The 411 I would not get too worried about. Clear it and see if it comes back, worst case you will need a TPS. Personally I have never done a TPS on an 850. Doesn't mean they can't go bad though, but I would think if it was in fact bad that you would be seeing codes out of the A2 socket as well.

122 in the Combined Inst. Cluster. Just clear it and move on. Check it again in a week or so and if it re-occurs (doubtful) then see if it is really effecting anything. Is the guage reading right? If it posts an ECT code in the A2 socket though, I would stuff a temp sensor and thermostat in it. One is right beside the other and it is actually a little easier to get the sensor out if you have the thermostat housing out of the way. So while it is out of the way throw the old one in the trash and put in a new factory Volvo thermostat and sensor and be done with it. If you get no further temp sensor codes then disregard all of it and carry on.

The 135 code is another temp sensor code, I am really suprised that you have not posted a 123 code in the A2 socket. I see them regularily and they seem to be caused by bad senders as often as they are caused by thermostats that hang open a little bit. In light of the fact that you have two codes for the engine coolant temp I would be inclined to just stuff in a new ECT sensor and thermostat.

Seized fan for sure. Just pull the little fan out from under the grab handle and blow the crap and crud out of it and re-install. Most of them are just bound up with dust and stuff.

A 122 code here is normal it refers to the fact that you have not exceeded 35km/h or no speed signal. Normal part of the diagnostics on this car so unless you actually have no speedo operation then just ignore it.

A 211 code here indicates that you have a short in the wiring in the steering wheel module of your SRS system. I would clear this and drive the car and see if it comes back. Is the SRS light lit in your car after you start it or does it go out? According to the blue book it should light up if this code is present. Anyhow clear it out and see what happens. If it returns you will need to see someone that has experience working on air bag systems, I do not know if your local Indy is up to that or not. It is a bomb afterall.

As far as parts go you are looking at around $200.00 on the MAF, $50.00 on the ECT sensor, another $20.00 or so for a factory thermostat and gasket.

Hope this helps,
Mark