I'm considering buying a townhouse in a downtown, urban area. House is approximately 4 years old and has passed an Engineering inspection with flying colors. No problems detected. However, yesterday I visited the property and noticed an overwhelming smell of sewer gas outside the townhouse... on the front walk and in the street. Really gross. Went by property again this morning. Smell still there but less noticeable. It is unseasonably cool and breezy today. The smell seems to be coming from in and around a storm drain located on the corner of the street about 25 feet from this townhouse.

Have not smelled anything inside unit, and inspector checked all toilets and plumbing. Any thoughts? Should I be concerned?

Some manhole covers have a gasket to seal them, typically on a grease trap. Your manhole lacks gaskets and will be a continuing problem which the municipality will have to address.

Yes there are several places around where I live like this. Some are actually swamp gas, (yes in the city)
But that doesn't sound like your problem, there are also two places where there are junctions of sewage running all the way from the airport, the city says by the time it gets there it is 2 to 3 days old. If you arnt sure don't buy (think about what it might smell like in the summer)
Maybe the city can help you, in my city they actually respond by e_mail pretty quickly.