If you buy them at say... level 30, then their attack and defense will be low. I've noticed that they don't scale to your level either. I could get to level 100, and it'd still be the same attack and defense at which I bought them. So, would it be better to wait on buying them at a later point and investing my UN points into weapon/gear/vehicles first?
You should wait and buy them when you can no longer find real squad to keep you fighting at your max strength. But remember you can upgrade them as well.
Yeah, I knew you could upgrade them, but there's no point in doing it until you're at a higher attack/defense stat, since it'll just update it to your level~. So if I have 60 attack on a Striker at level 30, and I upgrade it at level 100, it'll only go up slightly, I'm assuming. Which means, I'd have to upgrade it yet again at a later time too. Seems easier to just wait until I get my attack/defense where I want it for the long run before I buy any hired squad.