For servo I would say 150 in/oz at a minimum. I use the XP DS1015 and it fits very well. Had a savox TG servo in there and it required a little modification on the servo mount but I think most other servos would fit fine. For the price though I think the DS servo is the best deal.

Castle is not really a race setup, as long as the combo that you were referencing is a 540 can it will work but a Tekin RSX combo with a 6.5t through 8.5t (how fast do you want it?) would be better... or choose whatever race ESC is your favorite.