The cheapest gun to get and my personal favorite to modify is the Nite Finder. Anywhere from $3 to $5 gets you a single shot, decently ranged gun out of the box.
For a little more ($5 - $7) you can get a Nerf Maverick, which is a revolver type gun that holds 6 shots. It fires a little less far and you can’t modify it to be pocket sized like I always do to my Nite Finders, but it’s the most popular choice around my dorm.
Anything else I generally think is too expensive for too little gain, especially the Longshot. The Longshot requires special darts, so you can’t just grab some off the floor in an epic fight. I don’t know about many other kinds.
I’ll add more to this post once I find pictures of both guns to work with.
As for location, my all time favorite nerf gun battle was in a narrow stairwell. People from the bottom had to climb four levels of stairs and play single flag CTF. Making it up past even the first level was extremely difficult due to 20 people trying to fight 20 or so.
Fold up tables are the perfect obstacles, as they can be used as weapons depots or knocked down and used as barriers. Try to get a setting with lots of rooms and hallways.