you start with getting a wooden spoon, a metal dish, a paint stripper heatgun, and a few handfuls of green coffee beans. You stir the beans, while applying heat, and 15 mins later you have great coffee. Then you buy a 5kg commercial roaster off eBay and have over 300kg of green beans stored in your garage. Of course you could stop somewhere between those, but… nah, why would anyone do that.
There are many options in the middle quite frankly. I used the heatgun and wooden spoon method for a year or so, doing small batches most weekends. I then moved up to a small drum roaster in a BBQ and used it for many years to do great coffee in larger batch sizes. I then stumbled on the eBay roaster and bought it on a whim, I used it a bit but because of the size had to store it away from Sydney at my mothers house; it sat unused for much of the last 10 years, but then my mother’s house sold recently and I relocated it and started using it.
There are some “commercial” home roasters you can look at. Look for i-Roast, Behmor, and Genecafe. Check out this site (I’m not affiliated, however I did buy an espresso machine from them) http://www.talkcoffee.com.au/products/equipment/coffee-roasters/home-roasters/
There’s also a “HGBM” home DIY setup that is reasonably popular, also called “corretto”, see Forums - CoffeeSnobs, it’s a great re-purposing of things you may have already.
The BBQ drum was fabricated by someone specifically for coffee, and was under two hundred bucks if I recall. There are some commercial ones like http://www.rkdrums.com/ that are simple and don’t need you to find someone to make it for you.
and in the end, sources of green beans will be the biggest priority - you need to find a place where you don’t need to buy a 60kg bag of green beans of one variety but can buy a few kilos of multiple varieties so you can try them all. I was part of the coffeesnobs community for that reason - they would buy a few bags of green beans and split that across members in a few kilo lots with little extra cost above the per-kilo cost of the bag, so you got access to beans at the right price. Since then, with the big roaster I have kind of moved past that a bit but in starting out you really need a place you can get green beans in a few kilo lots - some full service roasters might sell you green beans from stock, but don’t expect them to all do that since beans are often limited supply and they want to offer specialised beans to their roasted customers first.
Good luck, it’s a fun journey and one that really does allow you to get the best quality coffee at a great price - my parting thought is roast less beans more often because fresh is best !