I've never rented one, I've instead made do with my beater circular saw with a diamond blade. The good blades are close to $100 but DeWalt has one now for around $40 IIRC. The problem with a skill saw is it will only cut about 2 1/4 tl 2 1/2 depth, at least the standard 7 1/4 inch model. You can then break off the remaining and the jagged line will be beneath the ground level.

The rental outfits I contacted indicated that they would charge me for how much of the blade's diameter had been eaten away. I think that's what they said, but there was definitely some metric for measuring blade wear and tear.

The DIY with a skil type saw method is crude -- with my beater, I'd sometimes push it with one hand and use a sprayer bottle to keep the blade wet and dust down, though with some blades this is not required. You'll want to wear a respirator either way, IMO, as any sort of stone dust is black lung territory.

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1986 300SDL, 362K
1984 300D, 138K