well i would go for the newer engine any day, im not a fan of mariner emrcury bunch, so wold go for tohatsu or yamaha, the tohatsus are basically same as yamaha, just couple of years behind, so both good engines, but obviously the nwer one will have more time left! boat wise, it depends what price you can get as to whether its worth going second hand or not, as i find that many second hand asking prices are not far away from the new price, and i have a boat to add to your list, have a look at a zodiacpro 7 man, its 4.2m long, and a 40hp would go very nicely on it, try TL HARVEY for this, think the website is his name! he does superb prices, bought my pro 9 man off him 2 years ago, and his prices were way below what anyone else could get any where near to! its a good little boat, and in my opinion way better than the bombard, build quality is good on both though, but not as sturdy as larger models, the hulls on the zodiac pro ribs are very good, and excellent in choppy weather, providing its not big chop though, simple fact of a shorter boat length, it will get hairy
you will find on the zodiac you have allot more room to move around than other ribs of comparable size too! Harvey fit suzukis, they do the others but, i have a suzuki 50hp and wouldnt change it for the world! unless it was a bigger suzuki! tohatsu ribs are good enough, similar in hull shape to the avon's, the lager models anyways, not sure bout the 4m lot, erm hope this helps!