Do you need one? Yes, probably. At least, I find mine incredibly useful.
If I could only keep one light, it’d probably be my ZL H52Fw. It’s not an exciting light and doesn’t have the awesome Nichia 219 tint, but it’s definitely my most useful and practical light.
The mule-style ones (H502*, H602*) have the widest and most-even beam, but almost no throw. The ‘floody’ ones (H52F*, H600F*) have a nice smooth beam but still have a bit of throw, and this is my favorite style… it looks a lot like it has DC-Fix on the lens, except it’s even smoother and has no artifacts at all in the beam. Or the base-level ones (H52, H600) have a traditional spot+spill pattern with throw. Useful while biking, for example, but not as great up close unless you only want to see a small area.
If you can find one, a H502d might suit your purposes… it’s about 5000K and higher CRI than usual (but still not as nice as a Nichia 219B). I prefer the diffused beam pattern instead of the mule beam pattern though.
Personally I find (and several others have said) that small is good when it’s strapped to your head. The AA or CR123 models are nice here. A 1x18650 light might be a bit on the heavy side. However, I’ve also heard great things about the Nitecore HC50.
As for vertical vs horizontal, get one which straps on in a horizontal position and allows you to rotate it freely. This solves the vertical aim problem, and you can do the horizontal aim by turning your head.
These are some of the tools I find most useful while modding: Soldering iron (of course), helping hands w/ extras, stereo magnifying loupes, headlamp, and tweezers.
Also, if you haven’t seen it, a few tips on ZL headlamp use: