Either of the PLUS chips would be best IMHO, the K6III (non plus) ran hotter from memory
As fond as I am of older technology (especially AMD processors) bang-for-buck you would most likely be able to upgrade the board, processor & ram for less than a now rare & exotic K6 II/III+ series CPU, however should you not be detered by this eBay has 15 such items listed in their International listings complete with extortionist shipping rates
current K6 III+ offerings
Alternatively there's a K62+ 550Mhz chip advertised for $10.99 USD with free delivery (surprisingly cheap) from China
HERE
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