Money. I think that's the biggest issue (seems like it always is!).
How much money can you spend on your roaster (and depending on area and EPA regs maybe an afterburner)? What features would you like? What are must haves and what are like to haves? What size do you think will be best for you? Oh, and when calculating what you can spend on your roaster don't forget everything else too -- grinder(s), sealers, scales, piping, wiring, bean inventory, bags, labels, storage bins, work tables, permits, etc., etc.
Back to the roaster . . . Now make a matrix of roaster features, and prices, and sizes and see what you can afford. Maybe you can get what you want, maybe you'll have to compromise a bit.
We started with an Ozturk 10kg roaster. Because of issues related to roasting small batches in a large roaster maybe we should have started with a 5kg, or even a 3kg, but then sales quickly made us glad to have the 10kg.
Hankua mentioned machines with "adjustable gas and air flow" and "an automatic machine might be what your after or one with a digital control gas valve".
You may want that control but it probably comes at a price. But maybe that means burning less beans while you're learning the ins-and-outs of your machine. Of course burning up some beans means that you are learning! Just be sure to apply the lessons.
Then if you are stating out and have a big roaster, but only need to roast small batches, you've got a challenge on your hands. This isn't a big challenge to experienced roasters but to newbies it can present problems . . . or opportunities to learn depending on your point of view.
Finally the bells and whistles you can get on your roaster are endless. But there are guys roasting with 20 year old machines, with no bells and whistles, and they're putting out a better product than folks with the most sophisticated roasters. So don't fall in love with all the technology, focus on getting a solid machine and then really working on the art and craft of roasting.
