Typically an inspection prior to an offer is called a pre-listing inspection and is done by the seller/seller's Realtor.
However, I just got back from three of my "WALK" inspections. This type of inspection is where I simply go with the Client and Realtor to the house and spend 30 minutes or so looking for very significant problems so that the Client doesn't have to tie up money in escrow on a house that has problems that he doesn't want to deal with. I provide no written report, just a verbal discussion. I usually do these at open houses, but Realtors know they can call me anytime to go look at a house before an offer is made on it. Saves everyone a lot of time and trouble.
As far as mold in the attic, that is quite commonplace. Typically it is not actively growing unless there is a roofing leak or plumbing leak.
Lumber to build our homes comes from the lumber mills as "green lumber." What that means is that it has a high water content, sometimes (depending on location) as high as 40% or so. While it is in transit, mold can grow due to that high moisture content, and if it is in transit in rain, or sits out in the rain at the construction site, again mold grows. I really don't think I've ever been in an attic that doesn't have mold.
The problem is when we start providing moisture to the mold and it begins growing, flowering, and releasing mold spores into the air. That's when we start having allergic reactions and the smell becomes quite noticeable.
I probably would pass on doing an air evaluation of the attic. I'd probably also pass on taking care of the mold in the attic as well, but that's a personal choice. I would be more interested in having a roofing contractor check the roof and flashing to make sure they are not leaking. Obviously, if you're needing to sell quickly, then addressing problems or potential problems beforehand is astute. Otherwise, virtually everything's negotiable.