The main advantage of the mods as I see them, are the increased adjustability of the detector.
If you already know the machine, you can tweak, and get a little more out of it in the right conditions. Coil size, gold size, depth, etc....
The stock 4500 is going to be just fine un-modded. Once you've found some gold with it, then you can make a better decision about spending another $1800 on your detector.
The main thing is getting out there. If you can't find anything with a stock detector, the mods probably won't help either.