I used to use 5gal buckets for my 3gal batches, works just find. I have since move to a stainless ssbrewtech mini bucket, at 3.5gal it has not needed a blow off yet, but I have not used any really aggressive top cropping yeasts.
I'd just get a 6 gallon fermentor. You can brew 3 gallon batches in there and you'll be set for when you inevitably step up to 5 gallon batches. Might as well get 2 while you're at it!
The fermentation of the beer makes so much excess CO2 that you can use a fairly large fermenter for the 2 1/2 gallon batch. I normally use a 6 1/2 gallon fermenter since that is what I started with when making 5 gallon batches. If you don't already have the fermenter the white plastic 5 gallon buckets work fine too and are a bit cheaper. Buy 2 or more so you can have more than one batch in the fermenter as it helps keep you from bottling too early just to empty a fermenter for the next batch.