If I'm correct in interpreting the OP's original request, I think it's essential that the roaster be able to reliably report the environment and bean temperatures. Those seem essential to me to the understanding of the roasting process. Without bean temperature you're just shooting in the dark.

It seems to me that rules out the Gene and the potentially the Behmor.

Well, the OP didn't specify that requirement in his first post. That was something suggested by another poster (#5). The Behmor does give you roast wall and exhaust temps, but you'd need to do a mod, such as this one, to get bean mass temperature:
Behmor RoR Artisan Mod and airflow control - with pictures

I'm not aware of any machines (except popcorn popper type roasters you can equip with a digital thermometer) under $600 or even the $1000 range that offer bean mass temperature measurement. The Behmor, on the other hand, provides a nice easy to work on platform for those who want to hack it for various functional "improvements." And as I said, there is a vast Behmor user community that posts a great deal of on-line information about machine use and mods.

Thanks everyone for the feedback super interesting. And sorry for the lack of response recently, as I have been lain low by illness and trying to juggle kids and work etc.

I have decided that I will start basic and go down a DIY route until I have the appetite and can justify the budget for something more fancy.

So currently I have bought a higher wattage popcorn popper, and will follow this guide to modding it, to split fan and heater control, and add probes for logging to Artisan.

Depending on how long before I kill it I may go with a souped up version with an airblower and heatgun after that.

Longer term, seems like I should have my eyes on a Quest or the Bullet, but have to imagine the Bullet may not be a smart financial decision for a while. Have also got my eye on the Kaleido Sniper, which would look cool in a garage, but not a kitchen.

I have the popper and checked it is working with some popcorn, so now I just need to get some green beans and various bits to mod it. I will definitely be back to read roasting guides etc to try and hone my process.

Thanks again for all the advice.

If I'm correct in interpreting the OP's original request, I think it's essential that the roaster be able to reliably report the environment and bean temperatures. Those seem essential to me to the understanding of the roasting process. Without bean temperature you're just shooting in the dark.

It seems to me that rules out the Gene and the potentially the Behmor.

I did want digital logging, and would be interested to explore moving to digital control off the back of that.

I had that under my control point, with the follow up that I am very happy to engage in some DIY on it, which has lead me to my current conclusions.