As far as what chainsaw to purchase, I would recommend something about 50cc. The Stihl 026/MS260/MS261, Husqvarna 346 or the newer variants, and others.
When you say 36" tree, that's a biggun but if it's one tree vs a size you will encounter on a regular basis makes a big difference. If you are regularly encountering 36" wood you are really into the 90cc area of saw size which would be the Stihl 066/MS660 and the Husqvarna 390/395 and variants.
If you have a one tree situation or a "might have a tree that size" don't let that impact your decision too much. A saw in the 50cc class will be an all around saw and cut most of the firewood size stuff most people encounter.
An 18" bar will enable you to buck up just about any size wood by making noodling cuts as you buck. Not the fastest but works for the occasional large tree.
I have more than 10 chainsaws at any one time ranging from 30 something cc to over 100. The 50cc saws do 90% of my cutting.
I favor Stihls and have a lot of those but also have Husqvarna, Jonsered, Poulan, McCulloch, Homelite, etc. The brand of saw you buy is not the most critical issue. The most important thing relating to chainsaw longevity is to use a fresh batch of properly mixed fuel and of utmost importance, keep your chain sharp. If you treat a saw like the typical homeowner does which is to cut up the broken tree, put the saw on the shelf for x years and then get it out and run it (or attempt to) on the stale mix with a dull chain, you will be adding that saw to the junk heap and getting a new one. Most of the "throw away" saws spoken of aren't really all that bad mechanically, it is the typical treatment they are subjected to. You can buy the most expensive "pro" saw out there and if you treat it poorly, you will have the exact same problems as the $99 saws. Treat the pro saw and the $99 saw well and they will both serve you well. Most pro saws don't get thrown away because they are so expensive to replace, they get fixed.
So if you are going to have one saw, the 50cc class is where you need to start. Then add a larger and smaller saw later if you find you could benefit from it.
