You should know at the outset that appropriate electric propulsion costs quite a bit more to buy, but quite a bit less to operate. Unless you are flush with cash go with a gasoline outboard.
First, let's talk about the meaning of appropriate as it applies to your application. I am not at all sure that a trolling motor would be the best choice as auxiliary power for a sailboat. It will work, of course, but a trolling motor is designed to move a boat slowly and quietly. A better choice for electric primary propulsion, IMO, is to find an electric motor designed for the purpose.
You'll need to determine what size electric motor you'll need. Practical Sailor looked at three electric motors in their article Practical Sailor Compares 3 Electric Outboards
. The Torqueedo 801L (a 2HP electric outboard) pushed their 19 foot Cape Dory at 4.5 kts at maximum output. That is realistic; I have a Travel 801L myself and it drives my dinghy (an 8' porta-bote) very nicely. I'd say 2 or maybe even 2.5 should be about right for you application and should give you reasonable speed even in chop and headwind.
Now that you know the motor size, what do you need to power it? The 2HP Torqueedo
runs at 24 volts. The 2.5 HP Ray electric outboard
runs at 36 volts. Clearly, you'll need a separate electrical system. The external battery model equivalent to the Torqeedo looks to be this one.
Torqeedo has an interesting page that argues strongly for lithium manganese oxide batteries. They sell such a large 24V battery product but you'll pay for it. I'd recommend an appropriate number of golf cart batteries to get the voltage you need (4 for the Torqeedo, 6 for the Ray). They are designed for the kind of current draw you'll be producing with the electric outboard.
Finally, there's the price. Electric outboards are still very new, rather immature as technology goes, and change often. Ergo the price is still high so the vendors can pay for ongoing R&D. The Torqeedo listed above will set you back about 3 boat bucks, the Ray will set you back 4. Also figure $150 per battery for really good, high capacity golf cart batteries and at least $500 for wiring, trays, hardware, and an appropriate intelligent charger. Yikes.
So now you know why I suggested you stick with a gasoline outboard.