Firstly may I welcome you to the site Wombat, I will try and answer your questions as best I can.
My boat is designed to be powered by 2 engines developing 500Hp, My first option was to fit Caterpillar Diesel engines as these IMOA are the best marine diesel engines money can buy, exactly I could not afford them. My next choice was to be Volvo Penta but after measuring the lengh and breadth of the engine compartment and working along with the Volvo Penta technical support team it became obvious no matter however we tried we could not fit 4 engines. So I made my next choice, but on reading a copy of the IMO MARPOL publication annexe 1997 concerning noxious emmisions at sea I had to cancel my order with Perkins, since air cooled engines was not really an option I had to take 5th best and put up with Cummins. I hope this answers Part 1.
Right now to tackle the second and IMHO the most important part of you question as this could affect a great many people not only on this forum but across the complete boating spectrum. There are at the moment a great range of powerfull, economical,and reliable second hand outboards on the market, but mark my words dont jump into this blind, hold onto your money old chap, work a few extra hours over the next couple of days and treat yourself to a brand new unit. With this you will get an operators manual and 12 months warranty giving you peace of mind.
I trust I have helped you and look forward to any comments you wish to return Bob