Yes, completely gut your engine room, scrub it and paint it, top to bottom, because it will never get smoky or greasy again. Consider removing and patching over all unnecessary through hulls. Wet exhaust? Cooling water? Those through hulls could someday bite you in the butt.
The ME0913 is a Brushless DC (PMAC) motor rated at 12kw continuous and 30kw peak at 96v and will run nicely at 48v. It should be enough to push your boat and it is not a particularly expensive motor. To go cheaper and still have the power you need, you would have to salvage a brushed DC motor from a forklift or something. Brushed DC motors are fine, but remember you can't have any propane or gasoline accessories onboard if you go with brushed motors. Any motor will require a controller which converts the energy stored in the battery to energy that will power the motor, and controls speed and/or torque by modifying that energy. A BLDC controller and a brushed motor controller are two completely different things. A brushless controller is more complex and will usually cost more than a brushless motor controller. And these are of course much different from the various types of AC controllers. AC motors are often used on larger vessels and higher performance cars but are a bit more expensive. Generally, brushed DC or BLDC motors are used in boats this size.
The best size prop is generally the biggest prop you can swing with a reasonable clearance between prop and hull or prop and skeg, whatever. The pitch is another factor. It is partly determined by the motor and reduction gear. You could have for instance a ME0907 6kw BLDC motor at 48V in a Catalina 27 coupled to a 2:1 reduction turning a 14x10 3 blade prop just fine, but find that a 14x14 prop is a bit too much. Or a 12x12 prop might work just fine, but a 12x8 not loading the motor as much as it should. It is all a big balancing act. There are tables and worksheets you can use to select components but in the final act, actual sea trials tell you whether you succeeded or not, and experimentation with different props or reduction ratios can fine tune your installation. Of course you can let someone else do the dirty work for you, and just install exactly what someone else optimized for his own boat identical to yours, and that certainly works, if you are not the experimenting type. Even easier, buy a turnkey kit and/or have a professional install your electric drive system. I am guessing the market is about equally split between professional installs and DIYs.
By 10kw I assume you mean 10kw/hr which is a more relevant unit when considering batteries and their capacity. It really depends on how you will be using your drive, and how you will be charging it. I have a Cal 2-27 which is somewhat smaller than your boat, and I currently have a 10.6kw/hr bank, a series of 8 6V golf cart batteries of 220AH capacity. A little Neanderthal but it was the cheapest way to have a decent size bank. There are higher tech solutions that you should consider if budget allows. Anyway this gives me a practical range of around 16 miles. Yes, under ideal conditions I could turn the power down to 100 watts and go probably 100 miles at .4kts but naaaah I dont think so lol. Then on the other hand I have a full speed range of about 6 miles, assuming a depth of discharge not exceeding 50% capacity. Going slow is second nature to eboaters but there are limits and you have to find your own comfort zone. When I have my planned 2kw solar canopy installed I will be able to go low and slow all day long without depleting the bank, and with a 2kw portable generator I know I can always get home. But my boat no longer has a mast and sails. If you are using electric only as true auxillary propulsion, i.e. just entering or leaving harbor, transiting bridges or narrow channels, anchoring, etc then a 10kw/hr bank should be plenty for propulsion. Consider whether or not you will also have house and navigation loads on the bank, though. Anyway, it is not so difficult to add to your bank and increase the voltage (within the specs of the motor and controller) or add a separate bank, which is what I will be doing this year. Note that increasing voltage may require a change in reduction ratio for optimal performance. A few hundred pounds of batteries make very little difference in a displacement sailboat hull. If the weight is an issue for some reason, go with LiFePO4 batteries which are much lighter, can discharge deeper, but cost much more.
You mentioned substantial currents. If currents of over a couple of knots are unavoidable, you may need a bigger motor. What was your previous engine power, and what do you think you actually needed, to maintain control of the boat in a current such as you anticipate? 12kw is roughly equivelant to 16HP. 30kw is roughly equivelant to 40HP. As a rule of thumb if an Atomic 4 is not too underpowered for your boat, then a 12kw motor is strong enough. Just make sure you have the range you need. Big difference between stemming a current in a narrow channel for 100 yards and doing the same thing for 4 miles.
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