Ah yes, the what can I get that will do everything question. The tool that does all things adequately will probably do nothing superbly. Having said that a pump or an inertia operated auto are the best bets. They are both cheaper and more rugged than a good quality SXS or OU.
Barrel length is personal preference, and pumps can be bought with replacement barrels for specific duties. I believe a more practical solution is to choose a barrel length suitable for a wide range of applications, and to my eye 24"-26" is very close to perfect in that they provide a decent sighting plane for pass shooting yet are quick handling for upland or big game hunting in thick cover. Choke options are limited to the choke tubes you wish to have, but an IC and modified would be a minimum requirement, (some guns shoot steel shot better with modified rather than full). If the model of gun you decide on doesn't have a barrel length that works, have a gunsmith shorten it and thread the muzzle for a set of choke tubes
Don't over look the importance of stock shape and fit. If you can get the advice of someone who understands shotgun fit that the best solution. This tends to escape me as the shotgun artists tend to like stocks that I consider too long, but thats why I shoot rifles I guess.
When it comes to slug shooting be sure to get proper sights for your gun. Rifle sights would tend to spoil the gun for other duties, so a scope or reflex sight that can be easily removed is the best answer. Because the gun is going to be used for a wide variety of applications, a good sling should be acquired, not only for use as a carry strap, but also as a shooting aid when slug shooting. In this respect the solid forend of the auto is more stable than the slide of a pump gun.