I also thought of going with "regular" used home heater but went against it and got garage heater instead.
Main issues with home heater in garage:
1. its big.
2. needs ducting
3. not optimized for garage heating (usually way too big)
4. used, no warranty, you don't know exactly what you are getting
5. expensive repairs are far more likely vs. garage style heater.
Actual $ savings are not that much as biggest cost is installation, unless you are licensed to do it yourself. This is also why I went with premium heater - not worth going cheap, it would decrease the price by like 10% at most.
All good points Tom. No doubt a garage specific heater can be the better option for most. A home furnace isn't the solution for everyone.
However, for me, the used furnace was the way to go.
1. It is bigger, but I don't find it overly big. Plus it's on the ceiling, so makes no difference to me. (I have 13' ceilings)
2. That's the only area I am not sure about. I don't know if there is a specific amount of ducting required. I have no ducting on the inlet side and 3' of ducting on the outlet, simply to direct the airflow downwards.
3. They will typically always be rated too big, but the rating is at max heat. There are usually at least 2 heat stages. I found a smaller one, rated for around a 1500~2000sqft home. I set mine to only run on low speed, low heat, so it works great for my 600sqft garage. I find it does not over-cycle set like this.
4. I wasn't concerned about warranty. I figured furnaces typically last for at least a couple decades, so I was fine with taking a chance on a 2yr old furnace.
5. If I can't repair it myself, it wouldn't be worth it to me, to pay $100/hr + for someone to repair either one. It could be argued that repairs are cheaper because parts are far more readily available.
That was my thought pattern and why it works for me. Also, I planned to keep my garage heated at all times, so going with a high efficiency furnace was the only way to go for me. And installation wasn't a concern, as I have an electrical ticket and my friend has a gas ticket.