I dont think the plane will be much more with the rcs 140 than with a ys 120 or 140, I have seen a bunch of guys flying with the large ys engines and it doesnt seem to bother it. If the wing loading is a concern then go with the DP Ultimate , it is a bit larger and a bit more wing area so i think you will be fine.
I know if i had to buy an engine i would go with the rcs for sure, I hate the mess of glow fuel. Well good luck with your decision, I would also go for the 5625 servos for the speed. I think the 5645 is like .24 on 4.8 volts, the torque is more than enough for a bipe this size
Good luck
Derf