I started off with a 60ltr with males guppies, i then bought a 2nd tank (a 40ltr) and we bought 3 female guppies. The day we got the females one gave birth (9 fry survived), we then upgraded the 60ltr to a 140ltr (to house the guppies) and then bought a couple female mollys, both gave birth within a week. We now have 4 tanks, 2 large tanks (140ltr and 80ltr), the 40ltr (fry tank) and a 35ltr (betta tank).
If you plan on getting males and females you will end up with more tanks. None of my tanks have cost me much, all bought 2nd hand (good old e-bay).
If i was you i would either get 3 male guppies or scrap the guppy idea and get a betta (they are great fun).