Good Morning G08.
First of all, It's absolutely critical to get a good home inspection when you are buying any real estate. In your situation, it's also very important to have an inspector that has the equipment and expertise to evaluate garden level spaces. There are many sources of water in a building, and a few are particular to the basement level.
A few things you should look for:
1. Is there an Overhead Sewer System? This is evidenced by a properly installed ejector pit and pump. The design of an overhead sewer raises the drain points in the basement to above grade and prevents sewer back up. The manholes in the street should leak back up before water comes up your tub drain in a properly installed system. You should also make sure this system is functioning properly and has been maintained. They are mechanical in nature and will eventually require service.
2. Proper Drain tile installation and pit. This is a segregated drainage system from the ejector/overhead sewer system. It takes water, attempting to penetrate through the foundation and routes it to a pit. The pit then evacuates the collected water and a pump pushes the water safely away from the foundation.
3. Does the building have proper downspouts and other roof drainage. If the downspout from the roof terminates at the base of the building, with no other control for the water, it's likely this water will try to find it's way into the foundation.
4. Positive grade away from the building. Water should not accumulate near the junction between the building and grade. All hardscape and grading should pitch water away from the building.
5. Leaks from units above. The garden level or basement is the final destination for all leaks in a building. You should make sure other residents pay attention to the condition of their plumbing and that the association deals with problems quickly. This includes, sinks, tubs, showers, toliets and appliances (washer/dishwasher/ice and water fridge/Central AC coils/water heater/humidifier). Roof leaks should also be dealt with quickly as this water is also headed downstairs. The association should have a go to plumber and roofer and a system for handling issues expeditiously.
6. Front and Rear Entry Drainage: The drains at the front and rear should be present, designed so that they are effective and don't clog easily, and tie into a system that doesn't allow sewer backup. You'll need to check these drains regularly to make sure they are clear of leaves and debris.
I'm not going to generalize and say all garden units are bad, or that finished basements are a bad ideal. It's all in how the space was designed and built. Typically, basements are built out to add value to a home, or add extra income or value to a building. Proper installation of ejector pumps, sump pumps, drain tile, vapor barriers, foundation drainage barriers takes skilled trades and cost good money and is much more cost effective when finishing the basement is planned for in the initial construction. Finishing space in an existing building will require substantial reworking of the existing drainage system. Because value is always a key concern, not everyone who finishes a basement or builds out a garden takes the appropriate measures to ensure a dry space.
Best of Luck and I hope you find the above information useful. If you need a qualified home inspection service, I am happy to give you a few. I will save the methods of testing for the home inspector community.
Keller Williams CCG