Wow, go away for a couple of days and a thread can really get away from you.
I agree that starting with a particular model is a great way to start a collection for a single manufacturer, and the Duofold is perfect for a Parker collection. It's good because they are plentiful (for the most part), so you'll have a good selection to choose from as you acquire them. Because of this, you should be selective and look hard at the pieces you're considering, because it's very easy to have caps and barrels from different pens swapped together, resulting in pens with very slight differences in color that might be hard to see unless you handle the pen. But it's important to the value that you make sure the cap and barrel match.
While a Duofold mini-collection may sound easy to pull together, it's not at all and will be quite challenging and diverse. Here's a quick glimpse at the styles and colors (I may miss some, so everyone feel free to add to this):
1. Hard Rubber: Red senior, Red Junior, Black plain Senior, Black plain Junior, Black chased Senior, Black chased Junior. All of the former with and without cap bands (wide and narrow for Seniors). Oh, there are Large, Medium and Small "Duofold" imprints of each of these.
2. Permanite Flat Top: Red, Black, Green, Lapis (white specks, blue specks), Yellow (yellow threads, black threads), Black and Pearl. Senior, Junior, Ladies and Special sizes of each.
3. Permanite Streamlined: Red, Black, Green, Lapis (white specks, blue specks), Yellow, Black and Pearl, Green and Pearl, Burgundy and Black, Seafoam Green. Senior, Junior, Ladies, Special, Juniorette, and Vest sizes of each.
This list isn't designed to intimidate you. It's meant to show you the diversity and what a great collection it'll be when you're done with it. It would be a very cool journey to be on.
If you go this direction, please make your first acquisition the Duofold Book by David Shepherd and Dan Zazove.
Good hunting!