Wow! I bet they are stunning, I think ill start off with just my one before getting anymore! (I don't think ill have room for any more tanks in my room!)
Well, I know of many people who do that, and have had lots of success, but the only thing different would be water changes, and you might need a stronger filter, but honestly I would just have one if I were you, it makes things much easier and you won't have to worry about the fish jumping the dividers
Personally I'd just stick to the one betta in there as well and not divide the tank. I myself don't like to keep them in any less than 5 gals, I like to see they have plenty of room Mine is in 7.4 gals and I'd give him far more if I could
Also, if you want a particularly fancy betta and are willing to spend the money, try aquabid.com (if in US) or Aquarist Classifieds & Bettaboys if in the UK I am aware ones on bettaboys tend to go for around £20 ($30) plus shipping, and likewise (if not more) on aquarist classifieds. May seem quite a lot of money, but when you see the type of fish you're paying for I'd say it was worth it