Existing track likely points to having a single recessed junction box in the center of the fixture, which means you're going to want to look for a surface mounted fixture, or "flush mount" as referred to in the consumer market. There are plenty of wrap-arounds, luminous bars, and the like that you can use for this purpose. A panel could be an option, though you have to be a little judicious.

what would be an appropriate size

That's going to be more a question of what looks good on your ceiling. You're probably going to want something in the order of 3500-4500 lumens, which is a fair amount of light, so having that spread out across a large lens is more comfortable visually than a tiny spotlight (similar to what you currently have). Assuming you have standard cabinets, I'd look for something in the 1x.4m size if you can. Longer and narrower would be okay, but might look funky.

watts / lumens / kelvins I have no idea what I'm looking for with these

I recommended 3500-4500 lumens (light output) above. That would put you in the 35-45W region. Color temperature can be more of a comfort thing; the idea that you need daylight is a bit of a misnomer. The best thing you can do is match it to whatever you have in the adjacent areas of your home. If it's incandescent, it'll be around 3000K. More importantly would be a CRI rating of 90ish to make sure you see your food correctly.

I'm guessing those are originally designed for office ceilings

Originally, the troffer was an office fixture that housed a couple fluorescent tubes. With LEDs, those became flat panels with no thickness, but were still intended to be laid into the standard ceiling grid. As you've noticed, it looks like some options include a bracket. Mount the bracket to the ceiling with some screws, then slide the fixture into place. Seems as easy as mounting any other light fixture.