Good deal my friend. An FFL transfer of the pistol will not tell you if it's stolen, only LE can check that by running the serial numbers to see if that particular serial number was reported stolen. You can have local LE check it for you if you feel it's necessary, otherwise, all you need to do is keep a record of when and from whom you purchased it. Deals like this happen every so often due to someones circumstances changing...you just happen to be at the right place at the right time - congratulations!
The G19 is an outstanding pistol - welcome to the "dark side"!, however, if it doesn't fit your hand (sometimes the location of the hump on the backstrap makes a difference, the G17 or G26 sits in my hand better than the G19) or needs, your can always use it to trade. The 9mm Glocks have the best reputation for longevity and, like other Glocks, ease of maintenance.