Hello everyone. (Pardon the re-post, just thought I'd get some more advice.)

About me: 20 years old. Most of my experience is hands-on and no official training outside of one photography editing/cataloging class I took in college last semester (I'm a junior this fall.) I have a steady clientele, but I mostly like shooting car shows and concerts. I am slowly transitioning to modeling and portraits with my connections in the local music and photography scenes. For me, this is a between high-caliber hobby and a job for me.

My kit: is a 70D with a 50mm 1.8 II, a 18-55mm 3.5-5.6 STM IS, and a 50-200mm STM
Additional accessories: Megagear Leather case, Sunpack 5858D, Sunpack Digiflash 3000.

Questions:

  1. Deal: A buddy of mine is selling me his 70-200mm f/4L USM Mark I (non IS-white) for $500. Should I take his offer or pay the extra $200 or so for a new lens and insurance?
  2. Flashes: Should I return the Digiflash and go for Canon official? What are the essential differences between all the flashes? Advantages of getting one that can be triggered untethered to camera? I normally don't use flash, can I compensate with faster lenses?
  3. Accessories: What accessories should I look into? Is the tripod I have good? Should I return it? What do you all think of shoulder pads or battery-grips? Lens hoods?
  4. Lenses: If I buy an L-lens for every day use what should I get? I shoot concerts and modeling portraits. What do you think of non-Canon lenses? For concerts/low lighting: Zooms versus Primes? What is the ultimate set-up for a bargain with my type of shooting? What is better; The 2.5-2.8 range lenses or the 1.2-2.0 lenses? I love my 1.8, but I can't zoom or get absolutely not-shakey images when things are moving. Is there something sharp, light, versatile and fast (that possibly zooms?)
  5. Incorporation/promotion: Should I get myself incorporated? Business cards? Agencies?