Is it reasonable to buy a new Apple Watch even though you own an iPhone 8?
iPhone 8 supports iOS 14 which is compatible with watchOS 7, the latest OS available. Therefore all the features of the new watch will be available without compromise compared to a new iPhone.
Your iPhone battery will not have any noticeable impact caused by an Apple Watch. Bluetooth is required for phone/watch  communication, which can use some small amount battery on any phone, and may be more of an impact on an older phone with worse battery life due to age.