Roughing cutters are great for hogging out great chunks of metal, rapidly. If you're going to be doing lots of deep pockets in metal, then it's probably worth having some around. Then again, if you're willing to take the time, you can do the same job with a regular end mill or slot drill, and the surface finish is better. You'd always finish off a rough-cut pocket with a proper endmill anyway, unless you are OK with the serrated finish.
You will be able to do everything you want to do with an equal set of 2 & 4 flute endmills. Being in the UK, I ended up buying a metric AND imperial set - I guess Stateside a metric set won't get used so much... It's probably worth getting duplicates some of the smaller end-mills, because they're the ones that'll break. 1/4" cutters used to die with monotonous regularity on my mill.
Don't worry about buying cheap - my set of 20 imperial cutters cost about UKP45 (~USD80?), and other than snapping the smaller ones, they're still cutting fabulously, nearly 18 months on (probably 'cos I've done nearly no machining recently

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I've never liked the idea of double-ended end-mills - too many finger-cutting opportunities, plus I wonder if the flutes that are gripped in the collet will go blunt.
I don't know what your collet/mill holder system is, be aware you may need a machined flat or a threaded end on your mills, which not all the cheapies may have.