I just brewed a variation on Lee's chestnut beer a few weeks ago. (Added 2 lbs of sorghum syrup, and lots more hops)
I mashed the chestnut chips on the stove in my brew kettle. (A lousy electric stove). With the stove on high, I was able to get it up to mashing temp, and then I put the cover on, dropped in a thermometer, and turned it down to low/medium-low, which was good to keep my temperature fairly stable for 24 hours with some slight adjustments throughout the day. Overnight, I put the stove on its lowest setting, and it only went down to about 140. I didn't wrap the brew kettle with anything to insulate, but I've read that others have tried that as well.
DME is no good for a GF starter. I went without a starter, but I suppose you could make a sorghum syrup starter.