Army building

It all depends on what you're wanting to achieve. Do you eventually want the whole line? (Don't lie to yourself) If so, getting boosters from the web is your best start. Once you notice that you're getting more extras than needs, switch to buying singles.

If you just want to collect specific nations, then still get boosters because you can get some good trade fodder. But don't buy as many and focus more on the harder to get sets. Use the singles market more. Or, check ebay for lots that have large numbers of minis in them