What is the right choice between NSDecimal, NSDecimalNumber, CFNumber?

I've read a lot about NSDecimal, NSNumber, NSNumberDecimal, CFNumber... and it begins to be a kind of jungle to me.

Basically, I'm trying to create a simple model class that will handle simple computations, like this one:

#import <Foundation/Foundation.h>


@interface Test : NSObject
{
    float rate;
    float amount;
    int duration;
}

- (float)capitalizedAmount;

@end

@implementation Test

- (float)capitalizedAmount {
    return (amount*pow((1.0+rate),duration));
}

@end

I want to access these methods and setters with their names as strings, since I plan to have a lot of other classes like this one, and I am only keeping a list of field to do key value coding.

// This is just the desired behavior
// This evidently won't work with the previous class definition
Test *obj = [[Test alloc] init];
[NSNumber numberWithInt:10]
...
float r;
r = [obj performSelector:NSSelectorFromString(@"capitalizedAmount")];

I understand that this is not possible, that performSelector: will return an object, and thus that capitalizedAmount should return an object. I've read things about NSInvocation and the relevant part in the Objective-C Faq on comp.lang.

I also understand that I should use NSDecimalNumber, but there is two things that I would like to know:

1. Are the memory overhead and performance loss acceptable for a somewhat more complicated class (only financial computations of this kind, showed in an UITableView)? I do not have much background in C...
2. Isn't it too fastidious and complicated to use functions like decimalNumberByAdding:? With Python it was easy to define __add__ to use operators with objects. Should I get float values from NSDecimalNumber, then do the computations and returns the result wrapped in an NSDecimalNumber? How would you deal with this problem?

I am looking for a simple and beautiful solution!

Just another question in the same area: is CFBoolean the object wrapper for BOOL on iPhone Core Foundation?

Thank you very much for your help!