Current Amplification
If I understand this correctly you essentially want to boost the current capability. While a transistor can do this, there's even better when you use an op amp and transistor to boost current. Shown below is a current boosting circuit that increases the output current of op amp via transistor and thus increases current capability from input of circuit. Why use an op amp you may ask. Op amp has high input impedance (much higher than a transistor buffer) which be useful to isolate output of DAC from your powerful load. Also the op amp with feedback will essentially make the output voltage equal to the input voltage. While I understand your concern with switching frequency ( I think you really mean gain bandwidth), there are op amps that have higher frequency performance.  Note that you can utilize a high frequency op amp, high current BJT, and voltage supply for op amp and bjt that is about 1V or 2V higher than Vin max.  OP amp can be LT6202 or THS4031 which has 100Mhz gain bandwidth for example but can be op amps with higher Gain bandwidth. 
Image Source: https://www.allaboutcircuits.com/technical-articles/how-to-buffer-an-op-amp-output-for-higher-current-part-1/
If your output needs to be positive and negative. You can use a circuit like this:

image source:https://www.allaboutcircuits.com/technical-articles/how-to-buffer-an-op-amp-output-for-higher-current-part-2/
For selecting an appropriate BJT:
Voltage requirement: If your system runs from 12V for example , a BJT with VCE voltage rating of three times the voltage is sufficient, ie 36V or higher. A few common VCE ratings are 40 and 60V.
Current Requirement: If your load will consume 500mA max then a transistor with at least three times that should suffice. For example 1.5A transistor will do.
Note that this is one way to determine a ratings for transistor and is based on providing plenty of margin for voltage and current.