If you have the model year correct and the serial is all "numbers" that would make it one of the six cylinder models and I would stay away from it.
If it's a "later" 1985 that was actually sold as part of the "brand new and redesigned" 1986 models, identified by an alpha-numeric serial and three cylinders then that is (obviously) a totally different motor. A 1985 build sold as an 86 will normally have a serial number starting 0A99xxx (zero, A, 99 etc)
In either case, a 90 "Mariner" is just a Merc that's painted grey and has different decals.
The three cylinder 90 Merc/Mariner was a very prolific motor and probably outsold every other 90 on the market by a 10:1 margin. The basic platform stayed in production in the US until 2005 (phased out to meet EPA requirements) but was produced and exported until about 2010. So there is a ton of replacement parts still out there (used) and there is good parts support (at least for now) from Merc. It's "little brother" the 75 Merc also shared the platform and was mechanically over 95% compatible.
If this is the six cylinder model I personally would consider it "quite" over priced at $900. It had rather picky Tillotson carbs on it that leaked at the best of times and kits for them are getting more and more scarce (Merc still sells "kits" but they lack all the parts you might need. As parts have gone No Longer Available, NLA, they have just left them out of the kit). At least these had a CDI ignition system unlike the earlier models in this run, so components are available. Likewise, servicing the water pump is still possible with OEM or aftermarket parts.
While the "value" can be very different from one location to another, personally, unless you want the "look" of the "tower of power" six cylinder I would stay away from that model and would personally peg the value at "maybe" 400 bucks if it was running near perfectly.
If it's the three cylinder model then parts wise it's fully supported for just about everything you might ever need both OEM and aftermarket. You can even find "complete carbs" in stock if you look around a bit.
It has a great ignition system and the platform survived virtually unchanged for 25 years, so they probably got it right from the start.
If I had use for a 90 I would consider a late-85 onward Merc or Merc Mariner without a second thought. Personally I would still have an issue dropping 900 bucks on a 32 year old motor, but again, I don't know what your local market is like. Ten years back I bought a "running perfectly" 1987, 140 Evinrude that had a new starter, trim motor and VRO pump for $550 which is about what I would be looking to spend on an 85 Merc if was similarly cared for.
There's my opinion and maybe worth what it cost ya
I have been working on "primarily" Merc's (25 horse and below) for somewhat over 40 years now but do own a couple of Evinrudes as well as my three Merc's. In Merc's larger motor's (over 50 horse) the only two I would own (2 strokes that is) is the three cylinder 75/90 or the six cylinder 135 - both post 1985. Others they tried to squeeze too many horses out of the platform and rebuilds were always in their future.