I dont mean to shine the light away from your original question, however have you thought of roasting with a bread maker and a heat gun? The set up is called a Corretto. There is plenty of info on the site to build your own. On top of that there are many people on here to give you advice about it. I manage to roast 450g of beans in one turn. batch takes between 11-16 mins. It cost me under $60 to set it up.
All that aside i like your idea of building a roaster, i hear there is alot involved. Dont let that stop you though. i think its a great idea and good luck with it.