Some suggestions.

Stick with an OS engine.

3151's are on the low end of the servo totem pole, they are glorified S148s. They will get you in the air but its cheaper to buy decent servos than it is to rebuild after a crash because sub standard servos aren't up to the task.

Heliproz web site prices on Futaba and OS are higher than they actually charge because of minimum advertised pricing agreements so it would pay to give them an email or call and find out what you will be actually paying. I know 9252 servos which are perfect for a Raptor 50 are about 69/each when you buy 4 of them and as stated the gyro/servo is only 185 or so.

http://www.heliproz.com A R50 V2 with an OS50 would get you pretty far with decent servos and a 401.