For left handed users the Keltecs are good choices because ambidextrous controls. However loading efficiently via the underside ejection port takes some training.
KS7 fits your budget. I like KS7 for size and weight, the carry handle is genuinely helpful, it's a neat package. Accuracy with slugs is excellent. The foregrip does not feel ideal initially. I adopted a c-clamp grip (wrap thumb to top of foregrip lip) and it works much better - no more beaten up thumb, easier to get down into the sights, improved push-pull recoil mitigation.
KSG doesn't fit your budget. Main feature of the KSG is the two magazine tubes. Not just the extra capacity, but being able to select ammo type - e.g. one tube of slugs, another of buck, etc. Easier to choke but no carry handle, more weight, more size, needs an optic for the rail, lots more money when all said and done. The special import 13.5" KSG is novel.
KS410 fits your budget. I've not tried yet, but maybe it brings benefits of both?
There is one semi-auto bullpup 12ga non-restricted - I have forgotten its name and I'm sure nobody will remind me. Lastly there is the Escort Bulltac which is a cheap KS7 rip off (pump). Availability is going to be a challenge on these. I'm not aware of anything else bullpup.