I do burgers on the main grill all the time, unless you're looking for "black and blue"
burgers you can probably even double up and use the upper grill at the same time. Steak is a little more difficult depending on the type (I like my sirloin at mid-high, but my filet seared), but typically the final/initial sear is done quickly so doing the bulk of the cooking, transferring to a plate (or second KK), and doing a quick sear assembly-line style seems pretty feasible as well.
However, I don't tend to cook for very large groups (I want to say 8 was my limit so far). What I don't like about the big gasser on the side is the propane vs. charcoal flavor. At grilling temps and with "junk meat" like burgers it's a bit more subtle, but it's still there - even if after the A1 or hot sauce it's mostly in my mind
Edit: The rotisserie is a neat toy, but personally I haven't found much use for it (which pains me because I know Dennis put a lot of effort into integrating it!). The tools you get for removing the grills are fantastic helpers, especially if you want to do a sear and remove the main grate while it's still hot. Beyond that, a GrillFloss for rough scaling, silicone gloves, and a long set of tongs are good additions.
You'll have to figure out for yourself how you like to light the thing. A charcoal chimney is good, many people like a MAPP torch, there's the propane option, and the little paraffin-foil starters. Many of these are best when paired with a chimney, so that's a good first purchase.