I wouldn't worry too much about the rpm avoid range, it is not really a problem and not worth buying a new prop for. As far as I know the only way to avoid it is to use a composite (wood or carbon) prop if that is allowed on your aircraft. We have an A3A and don't find it an issue, that said when we replace the engine we will go for a solid crank. To be clear the restriction is there to avoid damaging the prop rather than the engine, the vibrations that set up when operating in the avoid range (2150-2350) can eventually lead to a catastrophic failure of the prop which would cause a very "bad day".
The hollow section at the front of the crankshaft is to allow pressurised oil into the the hub of a constant speed prop which provides the force to change the pitch. If you are not intending to ever use a VP prop the solid crank is the way to go at engine replacement time. You could also buy a VP prop and solve the problem that way, be bit more than $5000 though!