Any standard servo (Futaba s3003, etc...) will suffice for the throttle.
I'd get a digital servo for the steering. You can save some coins and get a Hitec 5645 digital or spend more and get a Futaba, etc.....
Here's a nickel's worth of free advice. Throw away the HPI steering servo saver and replace it with a Kimbrough. You'll thank me later.