Hey there I'm soon getting a 6ft tank (187g!) and definitely want puffers, but i don't want to keep just a single one in there, so can anyone suggest a freshwater puffer that can be kept in groups?
I do keep them. Mine have never touched a plant. They will need lots of snails or some other hard food, though. I feed each 5-10 snails a day to avoid overgrown teeth.
Try a forum search on Colomesus asellus, there already are tons of good threads on them.
I'd rather let the UK residents answer the question about the retail price. Usually they are 8-16 Euro on the continent.
Lots of snail breeding (other puffers and loaches want snails, too!): 1 tank and 4 simple buckets. I feed different sizes, size of the puffers' eyes is said to be best.
Give a man a fish, and he'll eat for a day. Give a fish a man, and he'll eat for weeks.
Is bucket an unusual word (I am not a native speaker)?
I use 20 l buckets and hods used to mix mortar. Some plants, no filter, no heater. In summer they are outside the house. Hundreds of snails in each of them. Daphnia, cyclops and even Gammarus are often breeding in there.
Give a man a fish, and he'll eat for a day. Give a fish a man, and he'll eat for weeks.
Some call it a product of a mis-spent youth, I call it Rock n Roll ! 
(187g tank!!!!!)