In most situations it doesn't take much insulation to get good results. I have a $15 kids sleeping bag that fits perfectly over my kettle & burner. The zipper makes it quite easy to install/remove. With it I typically hold temps within 1 degF for a full hour. Only in the dead of winter will it drop more than that.
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Notice that my burner is inside the insulation. Its warm mass helps keep the temp stable.
If you do that, use very gentle heat and stir constantly. Otherwise you risk scorching the bag. If you leave your insulation closed for the whole mash you shouldn't need to add heat at all.
If you find that your mash temp does drop a few degrees over the hour, don't sweat it. There is
evidence that it will not make any difference in the finished beer.
The two purchases that should get the highest priority are fermentation temperature control and a grain mill, in that order.