You so do NOT want to try trace clipping with a tiny trimmer like the rechargable Wahls. I had a pair, they were fine for trims and touchups but not much else. The bare minimum clipper you need for a decent trace/blanket/hunter/body clip is something running A-5 blades, with as much power as you can get. Much preferable are the big Oster Sunbeams or Lister shears. I have an ancient set of Sunbeams that recently got refurbished again, and won't use anything else for serious clipping. They produce much better results, fewer lines, and do it in half or less the time of smaller clippers.

As far as clip patterns, unless you're conforming to a standard for foxhunting or something similar, you can clip as much or little as needed. I have two in light work this winter who got a mini-trace clip--bottom of the neck, chest, about 8" of shoulder just above the leg, and the underside of the belly. Flank and quarters are not clipped. It's just enough that I can get them cool in reasonable time, but not so much that they're in trouble if I'm late getting home to put blankets on.

Endurance people probably do the widest array of clip patterns. I saw one at a ride that was shaved from withers forward only. Turns out she hates blankets but loves her stretchy hood, so the owner clips what the hood will cover. More common are strip clips, and body clips that leave hair on the saddle area and legs.