2500 years on and still going strong. You can't go wrong with Greeks and Persians as a starter army. Many of the units will be useful in other armies if you spread out, as described. I find the best advice is to stick to two forces that one can provide a game without needing others to join in. That way you are always ready to set up and play. Of course with Persians and Spartans you start off with a perfect set of scenarios where one side can be outnumbered. Thermopylae can have a dozen Spartans (and some allies) versus as many regenerating Persians as you like - and a scenario can be weaved around that. My first game of Thermopylae was waged on the edge of a book shelf with Atlantic Greeks vs. all sorts of crazy stand in Persians, from Airfix Native Americans to Arabs. Then later Scruby 20mm were added.
The good news is you have a lot of options now to collect these armies.