First advice in this kind of market is not to buy until you actually have the free time to use it. Since it is now low cost it is best to get at least 600 dpi. However it is also important to test how long it takes to do a preview. My scanner takes under 6 seconds which is good. There are some old slow refurbished models on the market.
Image size is an important consideration for putting coins on the web.
I find images smaller than about 200 pixels on each side too small and larger than about 500 pixels too large. A US penny diameter 19 MM scanned at 300 dpi is 225 pixels. In general for this website coins smaller than a US penny is therefore scanned at 600 dpi and larger coins at 300 dpi. At 600 dpi even Fanams are about 200 pixels. The direction with which the coin is illuminated by the scanner is also important for high releaf coins since shadows could significantly change the appearance of the image.