First question back to you is, what truck is it going in?
* This will decide rear exhaust, side exhaust, slide carb, rotary carb, pull start, non pull start, etc.
Second question is will you be racing or just playing?
* This will determine how much motor you really need. The more high strung motors for racing will not last as long bashing. Also, if you want to race, you need to have a .12, at least here in the US, .15's are not legal in offroad.
All the motors you listed are good motors. I am partial to the any NovaRossi based motor. They seem to have the best reliability and have more tricks done to them from the factory.
As far as tunability, there isn't really one harder to tune than another. The Rossi motors come with an excelent carb. The OS motors have a very good carb also. Not sure on the RB. The only motor that I have found hard to tune are the original Trinity/Picco motors with the plastic carbs. Those carbs sucked. On the track, the RB and Rossi's are most popular around my area. Both are fast.
To pull start or not to pull start? If you don't need a pull start, don't get one unless you are looking for convenience. With a pull start you don't need to lug around a starter or bump starter and battery but your truck has to lug around the extra weight of the pull start when on the track. Pull starts eventually fail over time.