In my opinion, the earlier pre war models are the ones to look for; the Wood Grip and Tin Grip Juniors, although the Wood Grip model is very hard to find, the Straight Grip Mk.1, Straight Grip Senior and the Mk.11 Target, pre war Slant Grip Mk.1 and Senior. These are at the more expensive end, of the Webley Air Pistols, although the Wood Grip Mk.1 can often be found at a reasonable price as quite a few still survive, but the more modern models, the post war Slant Grip Mk.1 and Senior, the Premier, Junior, Typhoon, Hurricane, Tempest, are all readily available at reasonable prices.
Boxed examples can command quite a premium on the unboxed examples, and condition is everything.
These are the basics of any serious Webley Air Pistol collection, but there are some variations as well.
If you intend to get serious, The Gordon Bruce book 'Webley Air Pistols' might be a worthwhile purchase.
I'm sure others will have their own ideas on what constitutes the ideal Webley Air Pistol collection, but good luck with whatever you decide to go for.