Congrats on your Soling. I have yet to sail on one but they seem like a fun class of sail boat.
There is a community sailing association here in NYC ( SailNY - New York City's Community Sailing Association
) that has a fleet of Solings which they sail on the Hudson River. There can be 2 knot currents in the Hudson and their boats are all equipped with outboards as a safety precaution. The photos on their website do not show the motor mount or HP rating of the engines they use - that I could see.
I'd go with SailingDoggie's advice about a 3.5 HP engine.
The 'purists' should be largely silent as you have ordered new sails for her.