OP, I have been mastering and restoring music for over 12 years now.
I would highly recommend you mix and master on monitors first. Double check on monitors and triple check on headphones.
A fantastic entry level mastering grade headphone I would recommend would be the Senn HD600 or Fischer Audio FA-003. Those are the cheapest options I would recommend and the ones that will give you the most bang for the buck. Your chain will matter too! I would suggest a Benchmark DAC1 for amp/dac headphone or an objective 2/ODAC for a more affordable solution which is just as good, although I have yet to hear my ODAC but it supposedly measures better than the Benchmark.
If you want better, more expensive options, then I would recommend you look into the HE-500/HE-6 or Stax SR-007/SR-0009. Be prepared to spend a hefty sum on those headphones as well as supporting gear.
The best and most affordable solution, and one I use often, is a custom IEM. The UERM is excellent for mastering work and is extremely accurate and doesn't really require a huge amount of money for supporting gear.
If you have any other questions or just want to talk shop, then feel free to PM me