Reading the above information may be quite useful but there is one problem. A loaf of bread is available at literally every corner shop but unfortunately the same cannot be said about second hand marine diesel engines. So why not use the modern day technology of Internet and use websites for searching for such engines.
This would give you a lot of options in terms of make, model, condition and price. You can correspond with them and find out if they can ship the required model to your place. Even if not such an online research would give you a deep insight into the price levels etc which will help you to make a buying decision.
Also though size does matter and normally bigger is considered better, it could also be true the other way round since large websites might have too many options which means you might get confused and lost. So sometimes it is better to contact smaller websites which might have the time to provide personal and customized service to you.
Some of the sites which could be of help are as follows and it depends on what sort, type, model, size and power engine you are looking for.
Trans Atlantic Diesels
Gold Coast Power
In case you also want to buy a propeller for your boat, here is an article which deals with such stuff.