A BP will require an enclosure that is larger than 20gls... I think most agree that 40gls is the minimum. If you aren't set on cage size though, and willing to make the necessary modifications to a glass enclosure (insulation, foil top for humidity, etc.) one might work for you.

That said, they can have feeding issues.. I've had them go on food strikes and then only accept a different prey item, etc. You would need to get one from a good breeder (not a store) and request a juvie that was feeding well on f/t with a good feeding response. You would also need to setup the enclosure well in advance and monitor your temps and humidity so that everything was set before the BP came home (this really goes for all snakes).

I purchased a male western hognose as my 2nd snake. My male had serious feeding problems and I ended up rehoming him with someone that raised their own live feeders. He was previously eating f/t mice pinks and ate those for about 2 months when I got him, then went on months' long hunger strikes, broken up by the ocassional meal. I tried everything..braining, scenting, etc. nothing worked except live rat pinks. After those issues, I read accounts of others with males that had feeding issues and based on that, I'd only get a female very well-started (juvie) hognose feeding on unscented f/t.

I have a Kenyan Sand boa and they are generally good feeders. The only issue i have with mine is that they stay hidden most of the time. They are good snakes to handle those and generally docile. They also don't have the humidity requirements of BPs.

Of the snakes you mentioned you were interested in (kingsnake and corn) I'd personally go with either the king or corn... Both are better feeders and less picky about humidity than the others you are looking at. I would still get a bigger enclosure for them though (40gl) so that they have room to move around and explore.