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I've been looking at picking up a turntable. Most of the good ones I have been seeing for sale have been so DIYed, I hardly know if they are still the same turntable. I saw Thorens that looked good but the base and feet were both replaced and then lots of other retrofits.
There are few mods to turntables that can't be turned back to stock. Mostly they're as you mention, removing or changing the chassis bottom and/or feet and perhaps damping the platter and/or chassis. I wouldn't worry too much about mods; concentrate on condition. I'd also go out on a limb and say that in the Thorens, Rega, AR range there is little substantive difference in sound. Steve Brown and I did a comparo some years ago between his TD-125 and my Ariston RD 11; the similarities were more striking than the differences. Soon after I heard a P-3 at LSAF and it wasn't all that different in tone and texture, and that was unsuspended. The big story of our informal comparison was the different sound of the arms: his AT-1005 and my SME 3009-II. Swapping cartridges back and forth, the superiority of the Audio Technica was clear. Now that's what sits on my VPI HW-19 MK IV.