Setting linear units for Buffer distance from arcpy.GetParameterAsText()?

I am trying to run a buffer tool as part of script that I will both use as a standalone script and import into ArcToolbox. I'm just wondering, if I use GetParameterAsText for the buffer distance, how do I set the linear units? E.g. I want it preset so they only have to type 300 and the script takes that to be meters? Here is what I have so far:

# Buffer the Thames feature class to create the Flood Risk Area
in_features = "Thames"
out_feature_class = "Flood_R_A"
# Run the Buffer Tool
arcpy.Buffer_analysis (in_features, out_feature_class, arcpy.GetParameterAsText(5),    "FULL", "ROUND", "ALL", "")