I wouldn't worry to much about the depth of field with the lower values in Fstop
the further away from the lens de larger the depth of field becomes
this is only an issue when you're shooting from a very short distance
if you're shooting a 50m f1.8 orso out on the track the distance should be long enough to get good results
+ 'isolating' a jumping car surrounded with blur from the depth of field can make a great shot !
to my experience 50mm is a bit to small for RC racing to get good full frame shots from the cars it would be better to go with 24-70mm 2.8f or 70-200 2.8f however they are very expensive lenses
but they are sharp as hell and keep the low 2.8f when you zoom in
I would say, invest in the lens not the body, they will make your pictures much better then getting the next new megapixel body