I agree with the posts above. I entered this industry as an Arborist. I went to a vocational high school for Arboriculture. It is definitely something what should be trained by someone with experience. My impression from your post is that you have no experience and are not receiving training from anybody, as you would probably ask them these simple question.
I will answer some of your questions, climbing spikes I prefer Buckingham. Harnesses, I prefer Weaver, one particular harness in particular, but Buckingham is definitely the most common harness manufacturer. It comes down to personal preference. I know a number of guys who swear buy the same Buckingham model with the single but stap. I hate it and find it crushes my boys. They get into my Weaver with separate leg straps and they hate it and some say it cuts of circulation to their legs. The best thing to do is not buy your harness from a website or catalog, go to a local distributor who has models you can try on, particularly one who has a robe suspended from the ceiling, so that you can see how the harness works and feels with body thrusting and foot locking techniques. What is also important is how it feels when using the different D rings with a split tail, lanyards, presic loops, etc... This may not seem like a big deal, but these side D rings allow for better body placement when making cuts. I would much rather give up a little comfort when ascending or descending to have a little more comfort and security when making cuts.
All that said I strongly recomend havng someone with experience do the work and not try to attempt learning it on your own. Also check with your insurance, as most general liability for landscaping does not cover climbing, working on latters over a certain number of feet, or felling trees over a certain height.
Why do people not respect us as they do other tradesmen? Because every Tom, Dick, and Harry doesn't think he can be a plumber or electrician!