For firewood processing you can manage fine with one saw.
Make it a petrol one if money allows then you process the timber away from an electric point.
Training wise... Unless you plan to fell trees and head down the expensive road of insurance to do so, Maintenance and Crosscutting is all you need for processing firewood. In reality you don't need training at all for your personal use at home but to be professionally trained is clearly the very best use of any spare cash and will ensure you are an excellent user and maintainer of chainsaws. You may eben save the money you spend on training in the long term as you will be able to repair and properly sharpen your saw with the knowledge you gain. This will lengthen its life.
If money allows go for Stihl or Husqvarna and go for a beefy engine. 12-18inch bar. Try to go for a semi pro or pro version as the domestic ones tend to be nasty plastic these days in places that they shouldn't be nasty plastic. Yes a petrol saw can be heavy but if you're processing firewood you will soon develop the muscles to match and a heavy saw will be gravity assisted through the timber anyway.
PPE. Chainsaw protective trousers or chaps, chainsaw protective boots and a bash hat with visor and ear protection. HSE dictactes gloves which you will need should you go to be trained and assessed. In reality they are of little use as the protective material is in the rear of the left hand and they were designed and specified before the days of the front hand guard. The most beneficial element to them these days is to keep the hands warm which helps to prevent HAVS or white finger.
A good saw horse (wooden ones are best) either folding or one you have made is a great benefit and encourages safe practice. Metal ones may last longer but can be hard on chains if operator error in saw handling is a potential issue.
2 stroke oil and fuel when mixed degrades rapidly (1 -2 weeks) which can result in engine damage and failure. Stihl long life 2 stroke oil lasts much longer when mixed. Its why the repair shops are full of knackered saws and hedge trimmer in the spring... Folks get the machines out and use mixed fuel from last year.
Aspen is nicer to use but not essential.
Unless you plan to use your saw every couple of weeks through the year avoid bio or vegetable based chain oil.... It can set hard like gluey resin in parts of the saw exposed to oxygen and can clog oil ways if left unused for a while. It is a pain to clean off and un clog. Mineral chain oil doesn't go hard, is cheaper but is not as nice to the environment or you.
PM me if you want to chat anything through on the phone etc.. I've used saws professionally for around 18 years now and have been processing firewood for around 20 years.
Be safe and have fun
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