The first one I went after was the Crusader in black that was already listed as being retired and I had difficulty finding it in any local shops. Critical mass for him was 10 units before I could pause to catch my breath but I will stop at about 50 to back fill other lines like the Romans.
Then of course I needed a castle and so bought a 'carry along' castle for about $18 and then another for parts, then an 'Eagle Castle' and after reading the review comparing the Eagle and the Empire castle I decided the Eagle was good enough. But then they were the special for December from PlaymobilUS and I bought 2 more (for expansion purposes), then my wife bought me the Empire not knowing I had the Eagles! I bought the Rock castle so the bad guys would have a home, and at last I was sated. But wait! In one of the conference photos someone was using a western fort for the Romans. The hunt was on again and what of the Viking/Barbarians? are they not deserving of a place to be attacked? and so it goes.
The Eagle was the only castle I actually set up and it took my whole gaming table! As for my casting, I am still looking for sample pieces to use as masters but can see I need many hats and kitchen type items, for now I do this incidentally because there is so much to learn about the various lines and their history. I think my first project will end up being some new helmets for those Crusaders.
Regards,
Steve
Oh my, this parallels my own experience of the last 9 months!
I've always been more interested in the figures than the buildings, but having acquired a number of knights they needed a home, so off I went and got TWO Eagle castles AND a Knight's Empire Castle, fortunately both reduced, on the same day. That was quite an interesting trip home on the bus, and my arms are still 2" longer.
Exactly like you, I then needed a home for opposition, and off I went and got the Rock castle - but then discovered there was a bigger one, so I got that.. and then I got the Rock castle expansions from the acccessories range, actually, ahem, two of each.

Having finally set up the Empire castle (with expansions from the Eagle castles) I found I had about 50 knights to man it, so I split them into the Blue Watch (with blue lion shields), Yellow watch (yellow lion) and Green or Eagle Watch, comprising all the Eagle Knights. Of course then the watches weren't quite the same size, so I ha dto acquire more, or make customs from spare parts, to even them out.
The Rock Castle then followed along much the same lines, though I still haven't actually counted the Red Dragon knights or worked out watches.
All that doesn't touch on the Romans, Pirates and Western themes of which I have quite a lot of figures.
Regarding the Special Crusaders, I also bought a few of the Black Cross ones as they were already out of the shops, then discovered an older red cross one so got a couple of those (via eBay), and now there's a new one due out with a bigger red cross and a new style helmet, so as soon as he's available I'll be getting a dozen.
Then there's the Maltese Knight, who goes well with the Crusaders. They pop up on eBay occasionally, but they're not cheap.
And so it goes...
I have to set myself a unit size, maybe a dozen grunts with a couple of officers/NCOs, and then stick to it.