No, im saying, just using the stirrer as it is, may not do what you want it to, as Peter mentioned, you will have to mod the arm to get it to work right.
As for the entire PID thing, no that sounds great, Id like to see it.
Heat input and retention is important. If it heats too slow then you are more cooking or baking your beans than roasting them and besides for taking forever, the flavor will be ehh. Too fast and you scorch, tip, and other problems.
If the blower one could actually dance the beans that'd be perfect I think. One thing on thermostats, you have a pid .. thats good. you may need to take out the Thermo's they have built into those things, they may have one control, which is really just a very crude bimetallic clicker generally, and one overtemp to cut the whole thing off if it overheats. control it with your pid and you are good, be mindful of the power you pull through that pid controller, it may only like am amp or so, use external relays or I find that I like ssr's a lot better for power control.
Aaron