Okay For a guppy no c02 low tech tank. I'd get a 2~3 feet tank.. and then.. what i'd invest heavily on would be Filtration.
It is the most important part of the tank. since your main target is guppies, I believe having plain normal holland sand would do. No need for ADA soils. Whatever the tank size maybe, Please have a proper filtration system up as I believe it is the most important part of your tank.
Get a cheapo made in china canister filter, those may just be enough to fit your needs! For a 2feeter, I believe having an extra large HOB filter can work. While for a 3feeter, best go with a canister.
@cikar, with rm 300, what type of scape do you intend to do? I suggest having a nano ( 1ft~1.5ft maybe? ) to start off. With that, you would have enough money to spend on proper soils, maybe some DIY c02, and HOB filter. A 2feeter is possible too though.

And your "O"s that you replaced with zeroes are my my eyes @@
Drew
This post has been edited by chuppy2: Jul 22 2009, 09:28 PM