As you have one function inside other, you could do it something like:
EntityField isNumericLeftPart = new EntityField("IsNumericField",
new DbFunctionCall("IsNumeric", new object[] {
new DBFunctionCall("LEFT", new object[] { wheelbase, 3})
}));
filter.Add(isNumericLeftPart == 1);
The same applies to the second filter ( CAST(LEFT) )