In terms of garages flooding in the complex, I would check the seller's disclosure of the unit you are considering as well as speak to someone on the board of the condo association as well as the management company. You may also want to speak to a few of the homeowners who currently live in the complex. Finally, you may want to check with the town offices and see if they have any information on the flooding history in Powder Mill Square. This would at least give you a good head start in terms of assessing the potential of flooding at the complex. A good buyers agent should help you with this as well in terms of helping you cover all your bases when doing your due dilligence.
In terms of whether the property is a good investment, this also can be a difficult question as it depends on what your investment strategy and goals are, how the property fits into your personal investment portfolio, how long you intend to hold the property for, your cost of financing, the purchase price, as well as whether you are purchasing purely for investment or for personal use...
Strictly looking at the real estate market however, you can find some excellent data on the Andover market here: http://www.beangroup.net/massachusetts/market-reports/essex-
And all the condos currently on the market in Andover here: http://www.beangroup.net/Search/MA/Andover/condos?frm=agt&am
Good luck with your decision and please let me know if I can be of further assistance in your property search!