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The best way to avoid problems is to make the vendor tell you what V-series standards the product supports. Then use the handy glossary above.
If the abbreviation ``RPI'' occurs anywhere on the box, don't even consider buying the modem. RPI (Rockwell Protocol Interface) is a proprietary ``standard'' that allows modem makers to save a few bucks at your expense by using a cheap-jack Rockwell chipset that doesn't do error correction. Instead, it hands the job off to a modem driver which (on a Unix machine) you will not have.
Also avoid anything called a ``Windows Modem'' or ``WinModem'', ``HCF, or ``HSP''; these lobotomized pieces of crap require a Windows DLL to run and will eat up to 25% of your processor clocks during transfers.
All modems advertise mainly on DCE speed. Get a quote for max DTE speed too, if you can; make sure the DTE rate is enough times faster to handle your maximum on-the-fly compression (with V.42bis that's 4:1).
Multi-user Unix eats enough processor clocks that you want to be sure of good hardware buffering in your UART --- that is, enough of it to avoid losing characters between modem and PC if the OS is a bit slow responding to an interrupt (V.42bis in hardware won't detect this!). This means you want a 16550A or equivalent UART. If you're using an external modem, this is an issue about your serial-port board(s). If you're using an internal modem, the UART is on the modem card itself. So, when buying internal modems, ask what the UART type is. If the vendor says 16540, lose them.
Many fax modems come with bundled MS-DOS fax software that is at best useless under Unix, and at worst a software kluge to cover inadequate hardware. Avoid these bundles and buy a bare modem --- it's cheaper, and lowers the likelihood that something vital to your communications needs has been left out of the hardware.
Avoid ``Class 1'' and ``Class 2'' modems. Look for ``Class 2.0'' for the full EIA-standard command set.