Based on the information you have given us, no real estate agent will be able to tell you whether or not the condo you are considering is a good investment. Rather than focusing on the future salability of the property I would spend a long time considering the current market and how it relates to the property in question.
A very wise real estate mentor of mine once told me that "you don't make money when you sell a house, you make money when you buy a house." In other words, look for houses that already have an equity position built into them now, because when you sell you have little control over what a buyer will ultimately be willing to pay you for the house.
In any market economy limiting potential buyers will have a negative impact on pricing. To that end, I would say that a non FHA approved building would be a negative, but it wouldn't scare me away at face value. First, you can always work to get the condo approved by the FHA (somewhere in that dusty brain of mine I remember something about FHA doing away with the approved list, and working all approvals as spot approvals, but alas I can't remember exactly).
Second, it is possible that the negativity that the non FHA approval could have on the property may already be accounted for in the price. To figure this out, have your agent run some comps on similar active (yes active) condos that are FHA approved and see if there is a price disparity. They will complain that this will take a lot of work, but if they give you any lip, fire them and find someone who wants to work hard to find you the best deal possible. Remember that you are the customer and you deserve that.
Good luck in your search.
#1 Trulia Agent in MN