Yes, you can hunt deer with a 45cal round ball. I would look for a 50cal though. Reason is quite simple, tons of bullets and stuff out there for 50cal. Not so much for 45.
Don't get hung up on the 1/2 lighter thing. What you want in a flintlock is reliability and if you're new to muzzleloading, you're getting ready to jump into the deep end of the pool with no swimming lessons. One further thing on flintlocks, and read this carefully, THEY DO NOT PLAY NICE WITH BP SUBS. You will need real 3f bp. The pan can be primed with 3f and you can also use it in the main charge. Swiss is best and available as is Schuetzen. I was running Schuetzen in my flintlock last weekend with no issues.