Any higher throughput speeds require 16550 UARTs. Linux doesn't care whether your modem is an internal or external model, as long as it has a serial port designation that it can be addressed through. The choice between internal and external modem has become one-sided lately. Five years ago, the internal modem was the modem of choice. Most name-brand modems now are external, probably because of easier upgradability.
Most manufacturers offer both types of modems.
One thing to consider with many new modem models is their software upgradability. Some modems allow you to load new versions of the controlling software through a DOS session. You obtain the new software through a bulletin board or on-line service. This capability reduces the obsolescence factor considerably. If the modem you purchase is an internal modem, you must open your machine and add it in an unused slot.