When I bought my first rack (which i did buy a full rack of single cams that I had saved up for), I focused on a size range (BD) of around a .5 – 3. A single rack of that plus a single rack of nuts got me started super well. I led around on that single rack for far longer than probably I should have, then started buying doubles one at a time as I could find them on sale. I have a mix of regular BD, metolius mastercams, and really love aliens and wish I had more than one (that I bootied haha). The ultralights are sweet, but I have done a ton of alpine climbing with my regular old BD camalots.
My must- have at the moment is two fold: first off, do yourself a solid and buy a rack of nuts from DMM (the wallnuts). Their alloy is wonderful and softer and bites rock really really well. And second, if you are brave enough for the learning curve, I would just go buy the DMM offset nuts (both the offset wallnuts and the peenuts is what I have). Honestly, I rarely use a regular nut anymore. This does lead to some swearing by my seconds who are not as used to cleaning them, and they definitely can get stuck much easier, however especially in granite rock, you are never dealing with a parallel constriction and they just fit into way more spaces than they seem like they will!