I'm not sure if the trim will work without the kit, it usually would snap into it. A double din leaves no gaps, my installation looks fine, I used the OE brackets. In my opinion a double din will give the cleanest look, a single din with a kit shouts that it is aftermarket, double dins will blend in better and look more like they were meant to be there. I have a Kenwood unit, and I highly recommend Kenwood, I've always used them for headunits.
http://www.sonicelectronix.com --This site has the cheapest prices around, they are who I deal with and they offer free installation kits with the purchase of a headunit. They are very reliable.
http://www.crutchfield.com --Here is another reliable site, although they are a bit more pricey, but I'll let you make the decision.
Shop around a bit. I have a Kenwood DPX-301, if you want to check that out or any other Kenwood DPX series headunits.