If you want wider on FF, for cheap, not very fast, but cheap
look at the FA-J 18-35.
18mm on film gives you a 90° FOV (approximately), and that's soooo cool
It has almost no visible field distorsion, like UWA usually have. In fact, i think it has less distorsion than the 18-55 @18mm.
i'm on a seat, and my knees are just out of the picture by few inch
I found this lend for 40€ on ebay, few month ago, and that's almost my most used lens on film, when i'm traveling / discovering places alone. You will tell me "no time to travel" i say ok, but with a flash that you can bounce you can have some awesome familly picture with the whole familly (ie : 20 people on the frame on a single line
)
Baronite gave you the best advice : look at how you practice photog, what you shoot most or enjoy the most, and build your whole set around it.
I used to have around 15 lens, now i'm at 10, and i still have some i don't use a lot ...