Personally, I think those Kodas you were initially talking about would be a good choice in that price range. I personally haven't ever heard DLS's so I couldn't honestly compare IMO, but THey are a good solid fairly flat speaker that would do you well.
As for the size thing. I'd run 5's ONLY if it were part of a 3-way setup and they were going in kicks with larger midwoofers in the doors. 5's are very good sounding, but they struggle to get down to the 80hz range where you want to be to keep all of your sound upfront. Theoretically below 80hz is non-directional, but I honestly seem to be able to hear sources down to about 70 or so. The reasoning is that eventhougha nice SQ sub can play those higher notes cleanly, you'll still hear the bass coming from the trunk. instead of it seamlessly blending. Eitherway, a 6'll fit easily in your doors, there's no space issue to speak of, so you should be alright.
As far as HU's that Nak is indeed sweet, and I nearly bought one, but the MP3 thing was just too tempting for me. I can't live without it. But yes, you're right, you've prolly got ahwile til your kenwood craps out. So you could save some money there and get everythign else inplac,ethen just plug and go... As far as the RCAs are concerned, it's only really an issue if there's any tweaking you'd want to do from the HU. Most amps have a line-out anywhichway, so you can "daisychain" them if you had more than one. Also I wouldn't rule-out the possibility of getting a 4 channel amp and running the front two channels to your components and the rear's bridged to a sub, this is a very common simle SQ setup and does many ppl quite well. Unless you intend to bi-amp components or something it may be a more logical choice and better use of power.
Oh, sidenote, but something you should also consider, depending on the power you intend to push, is pulling new wire to your doors, and dynamatting them. Sound deadening on the doors makes a HUGE difference.