Hey,
please make sure you motherboard does not have a Realtek NIC, if it does, there's a high change that it will not work. In that case, you should get yourself an Intel NIC.
Yes, everything over 32GB for the boot drive is wasted space, since you cannot use the boot drive for anything else then the OS. Apps are typically installed on a pool, this would be your HDD pool if you are not going to create a SSD pool as well. So your 250GB SSD is fine as boot drive, if you don't have a smaller one.
Additionally, there is no write cache as other NAS OS might have, there is only a read cache, which doesn't offer a lot or any benefit with your use cases. So no, you don't need a cache drive, and big boot drive is just wasted space. 🙂