First you have to have a serial interface encoder and tachometer. Those should be sufficiently expensive, where it would pay to use the normal variety and a PLC. A DAQ card would do the tachometer and some will handle a quadrature encoder. I suppose you have to let us know if you actually have a serial encoder and tachometer or if you need to interface the normal items.
There are Ethernet remote I/O modules available which have high speed counter inputs that may work as encoder inputs (e.g. one of the Advantech 6000 modules may be suitable). They could be read via Modbus/TCP. Since any other I/O required could be handled the same way, this might be the most convenient solution.
