Hi there!
I dunno about 'economical' as things never pans out that way. What I can suggest is having a firm idea of what kind of army you wanna build. Then all 'economical' matters become secondary.
I love an aspect heavy army, my old list had banshees, fire dragons, warp spiders, dark reapers and dire avengers. They are fun, compact, but very glass-cannon. So you gotta play smart, and minimise risk.
In terms of HQ choice, a Farseer seems like a no-brainer, as he can buff the effectiveness and damage output of your glass-cannon units, but that's up to you.
If that's something you wanna do, then you can consider an aspect-heavy army. Cheers.