Best all around engine in my opinion is the Novarossi (Rex, Top, Crono, JP) P5N, RS5, B5N. Expect to pay around $279 more or less.
Best engine for the money. Novarossi N21B- You can find em for $150-200. www.nitrohouse.com is running a special on them. And PDLRacing1 on ebay has a few he will let go of cheap if you talk to him.