I’d recommend the base set for starters. It has a lot of core units (M4A1 Shermans, Humbers, SS-PGs, very good commanders, etc).
Also, it’s always better to buy cases if you can, rather than buying a cases worth of boosters individually - you’re 99.99% guaranteed a better spread of units, eg every case I’ve gotten has given me 4 of every common in the set, 2-3 of every uncommon, and usually 12/15 rares, though occasionally there’s a double.
Alternatively, go on eBay or to your FLGS that carries singles and just buy what you need. I prefer cases as it gives you the basis of what you need, and often extras for traders, but I do use eBay to get the numbers of some of the units I want.