Hello,
A driver transformer for a MOSFET should have certain properties such as:
Minimum leakage inductance by using 1:1 turns ratio, a gapless toroid core, and bifilar windings (trifilar for push-pull).
Minimum number of turns to reduce capacitance.
Cores with small diameters and narrow hysteresis loops to reduce magnetizing currents.
Low eddy losses and high flux capacities at the operating frequency.
Driver transformer example: Magnetics Inc. core 52485-1D, winding two windings of 32 turns each, two in hand for a bifilar winding, using one as primary and one as secondary of course. For push pull, three windings of 32 turns each three in hand for a trifilar (one input and two outputs).
Opto coupler designs are even simpler if you dont work with transformers or inductors too often. Remember darlingtons are just two transistors, and a transistor can be driven fast or slow depending on how it is used. A transistor that is driven into saturation is slow but one that is not is fast. A darlington is made of two transistors therefore if it is driven into saturation it is slow but if not it can be fast. It's not the darlington configuration that slows it down it is the way it is driven.
Using a logic opto coupler might be a good idea too, along with a driver transistor or two.
The power supply for the opto coupler drive circuit can be easily made from a transformer, rectifiers, filter caps, and even a small voltage regulator...it isnt that expensive really especially if you only need one. If you need four (sometimes used with bridges) then you use a transformer with four windings, four bridge rectifiers, four filter caps, four small three terminal voltage regulators. The windings are isolated so they help to make up isolated power supplies that can be ground referenced to the source(s) of the MOSFET(s) to be switched.