I'd actually build up your paints as and when you need them rather than buying a whole load all at once. When I switched from enamels to acrylic I thought I was going to buy the whole range that Foundry then did. I'm glad I didn't, instead I tried a few brands I'd read about and decided on which suited me the best. That happened to be Vallejo. What mattered to me was:
1) Quality – coverage, thin and mix well
2) Range of colours including some WW2
3) Cost
4) Availability
Into the last two brackets comes the fact that I can drive a few miles and buy single bottles of Vallejo as and when I need them. Not so with any other brand – may be different depending on where you live of course.
Next I bought a basic selection of about 15 colours that translated from my Humbrol palette. Over time I added about another ten colours as I stumbled over the need, then later some specific WW2 shades. Now this small selection is fine for me as I mix all my colours anyway. The big question is how do you want to paint and how much you want to spend?
If you want to use the tones the paint maker suggests then a range like Foundry will suit you with its triad system. If you are into mixing your own colours then the added expense of Foundry is simply not worth it.
My own view is that Vallejo make the best paint and that is where I'd start. They do boxed sets and you can buy individual colours as well. Very widely available. Over time I've added a couple of blues from other ranges.
If you go and buy the whole range from any make you'll end up with some you never ever use, and may be quite a lot you never use. For example, I only have one red and I use that to mix all the other reds I need. Saves lots of money and I can mix exactly the tones I want. I concede that painting like this is not to everyone's taste.