I have a Tohatsu 20hp as a kicker. Rock solid -- quiet, starts easy. 3 seasons on it and zero complaints. Tohatsu makes all the Mercury and Nissan engines 40Hp and lower (for Merc).
The 115 I believe is a 2-stroke TLDI engine. Supposed to be very good, with an excellent power to weight ratio and extremely low maintenance. Just make sure you have a good service centre close by. Loads of outlets to get a Yamaha, Honda, Merc, Evinrude etc serviced, but Tohatsu might be somewhat limited.