Willard and zodiac are the most common as they are government surplus and are only $25-50k used vs $100k+ new (I'm just guessing they might be $200k new). A huge majority of them are diesel boats but again they are built with the most robust components. I haven't heard of issues with them beyond normal maintenance. Remember built for government, money was no object.
As for outboards I am building a zodiac with a single 250. I think I have seen 4 others that have a single. 1 has since been converted back to twins. 1 other is just north if you. Search here for zodiac with Yamaha 300.
There is no easy answer to single vs twin. There are many many many pages of discussion here and on all the other boating forums. I went single as it's less expensive to buy, maintain, and more fuel efficient.