Well i still need help on this as iam still undecided , If i go for an out side trickle filter iam going to need a pump that has a head height of at least 3m to get it too the top tank, and that means i ll have to have both tanks connected which is also undesirable incase of a deasease.
The other thing i was thinking of was putting in an internal overflow box, I havent got the tanks yet and i was going to make them myself, if i make the tanks 51inches and use 3inches for the overflow box It still has 4ft of swimming space. The only thing is you said i would need 250bio balls for this tank maybe more with the added overflow box. The room in the box would be probably 12'x3' its 18'wide but id use 3 inches on each side for the overflow and 3 inches for the pump to get it back to the tank, leaving 12'x3' inches of room for bio-balls. I dont think this is enough room to fit 250+ bio balls although i haven't worked it out i dont think it would be.
The only think i can think of that looks like it would work would be a trickle filter but this would mean the 2 tanks would be connected, so if i had to coonect the 2 tanks how would th trickle filter work??
Iam guessing the overflow from the top tank would go into the bottom tank and then that would overflow into the ticklefilter/sump and from three it would be pumped back into the upper tank. Iam understanding this correctly?? Iam so lost, i dont know what to do. The internal box seemed like a good idea because i really want it to be in the tank, and it would be easy to construct, its just a matter of will there be enough room to fit 250+ bio balls. Even if i run this overflow system and then another of thoughs DIY internal cannister filters (as in post above) this might be enough filtration.

Some please guys i am racking my brain here!!
I think in the end i might end up with a fluval 404 and a couple of sponge filters.

Cheers
Scott