Lol, as it happens to be i think i got the biggest alfa romeo 155 model car collection in the world.
(All UT models 1:18, all minichamps 1:43, all herpa 1:87, all antonette43 1:43, all HPI in 1:43 and in 1:18, lots off BBR 1:43, lots of SC model 1:43, most trackstars 1:64, some wierd toys, 2 unknown maker toys, some 1:18 dicky pullback, most 1:43 remember kits, the glass daum 1:24, most onyx 1:43, and the ones i am forgetting at the moment.)
I did try to get that one as complete as possible, and got a very long way in getting them all. (Bar a few, and i don't add any new ones at them moment.)
But that is not all that i collect.
I do agree you should keep yourself focused on what you want to collect as there is way to much available to buy it all.
So i got some parts of the whole market i like to collect, and i like to keep those parts strict and make them as complete as possible. With the room i have available i am pretty much "complete" altough some models will be added when i spot a great one.
And that is just my way of collecting, if you just want to buy whatever comes your way you are free to do so.
But getting at a point were you got completeness will be much harder.