Another alternative to driving the existing propeller is the Mastervolt Podmaster
http://www.mastervolt.com/press-releases/the-new-podmaster-from-mastervolt/
A conversion of a classic daysailor with no auxiliary to this system was outlined in the most recent edition of Professional Boatbuilder. For this application the Podmaster was perfect since there was no engine compartment to put even a motor and very shallow bilges. The advantage in your situation is that the pod system is engineered as a unit. You don't have to re-engineer and re-specify propellers, etc. Just plug up the shaft log.
With the pod on the outside, this also frees up the old engine room space for easier retrofitting of a battery bank. One of the most complicated parts of retrofitting a boat to electric is finding places to put the batteries.
The same article notes another conversion where professionally installed the cost was $17,000 and estimates that this is $3,000 MORE than an equivalent diesel. Not that your installation would run that but illustrates that a proper electric install is not going to save much if anything over a traditional repower.
There was an article a few months ago in either This Old Boat or Sail that outlined converting a boat to electric drive using the original propeller. It was on how to do this on the cheap. However, the author appeared to be skilled to more than the shade tree mechanic level.
Also, as Nigel Calder has noted in several articles, there are significant hazards with available fault current and other issues that are associated with newer batteries (lithium) or just the scale up from an A/B battery set to a multi-battery bank.
As a minimum, adhere to ABYC standards and work with an ABYC technician certified eletrical technician. /And even current ABYC standards are really not designed to address these larger banks. These standards are in active development but since the industry is still in the infacncy of these electric and hybrid vessels the standards are chasing a moving target.
And make sure you are realistic about the application and your expectations. People frequently overestimate how much they can get out of a battery bank. In a hybrid boat presentation I gave, I used the Chevy Volt as an analogy. The state of the art batteries in this car weigh about 700 lb. Electric range is 30-40 miles. MPG once the engine kicks in is in the 30 mpg range. So this 700 lb. of battery has equivalent energy to about a gallon or gallon and a half of gasoline or diesel (we're talking just rough ballpark numbers to illustrate).
Envision it this way. Would you head off on a cruise with just a gallon or two in your tank, especially if even Sea Tow can't supply you with a gallon or two of emergency "fuel" out on the water? (Your electric "tank" size would vary depending on the size of your battery bank). If your answer is "yes" than you are a good candidate.