Not sure where to go with this one but...

I have a '95 850 w/ a broken tachometer/gas gauge/speedometer. The odometer and turbo gauge are still functioning fine. I have no clue what to diagnose or anything .

Would replacing the instrument cluster be a good idea? What steps should I take from here?

 

Pull the cluster and make sure that the plugs are all securely plugged in. something unplugged could cause what you're talking about.

There are several good writeups here about removing the instrument cluster.

 

http://www.matthewsvolvosite.com/200...er-fix-how-to/

That will give you an idea about how to get your cluster out...

I just did the whole odometer gear fix. I hate electronics because I always feel ill screw something up, but it was very easy...

when removing your vents the ones on the front of the dash you turn them down and pull and they will pop out. the side ones there is a little cover, then a screw under that, and then they snap out.