Even if you are mining to get some coin - whatever it is. You can always mine ethereum - get more cash this way and then convert it to whatever coin you want to have/keep.

If air condition does not blow directly into rig then it is ok.
Also GPU can easily run at 75*C. The Max temperature for most GPUs is 90*C and this is too much usually.
Also fans in GPU should not run on 100% so if they are running at 60% then you are all set

I had air conditioner that wat takin 5kW and cooling with power of 15kW.
Rigs that were in that room were taking about 25kW, so air conditioning was cooling but had not enough power. Room temperature was 30*C.
They were running or a 2 years in that conditions and they have totally no problem and running even now almost 3 years. GTX 1070 8GB.

It looked like this: https://youtu.be/F9_2WBalUF8

And you are right, it is clean and without dust.
Currently i have air cooled GPU farm and the dust is there
Actually dust is not bad, the worse are those flying worms which always get somehow in