time to repower

[nipper]

I received unfortunate news from my mechanic; my mercruiser 4.3 litre V6 is toast. I am seriously considering adding a bracket and repowering with an outboard. I have been on ebay, and find that there are some pretty new outboards (some new leftovers too) at prices I can afford. I am wondering what horsepower would be best, as well as whether I should go with a 4 stroke or not. Also, I do not know what shaft length I would need. Any advice would be appreciated. Thanks.