Welcome to the forum!
Sounds like you've got a great starting point for both a Rohan AND Haradrim army from a warriors point of view
Which one you would like to expand on first is really down to your gut feeling. The best advice I was given in here when first starting not so long ago was to go with the models I liked the look of first, then go from there.
I'm yet to try my Rohirrim our yet, but by many accounts they can be great fun to play, if not slightly difficult to master and be competitive yet.
Your next step is to get the sourcebooks - kingdoms of men for Rohan and fallen realms for the Haradrim. This will give you a great insight into how to build your force. The warbands rule means for you to get started, you need a hero from the same army for every 12 warriors.
So maybe an Erkenbrand and Eomer for your cavalry and Grimbold and a captain for your infantry and you're good to go
Good luck and keep us up to date with your progress
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