Okay, a word of advice, don't waste your time and money with any of the Warlord Nap plastics, as they aren't good at all. Often historically inaccurate and the sculpting leaves a lot to be desired (hands are often like hams!!!!!!!!!).
The Victrix figs offer more variety but they take a lot of work to whip into shape.
If your on a budget and want good figs then I would go with the Perry Nap range, as it is a good starting point and covers a lot of the main powers in plastic.
Have fun!
Of course, if you want the best detailed Nap figs, I would go with Avanpost!