I might add that Dakotas from 97-00 have different interiors than that of the 01-04. (One of the reasons I went with an 04.) Although there are some minor things they left out of my 04 that I have since re-added such as the ash tray and second cig lighter, they did add the 4x4 switch on the dash in 01 instead of the manual shifter on the floor.
The quad cabs are also very nice to have if you don't require the long bed. But since you're coming from a car, I imagine that's not a big deal to you.
My truck (2004 4x4 Quadcab) gets on average about 15mpgs according to the read out, but when actually calculating at the pump it's usually closer to 14. That's also mostly highway as I live 30 min from work.
I would advise you to go for an 01-04, but that's my personal preference because they have a newer interior but still the same sexy body style. The 05+, to me, are ugly as hell. My buddy bought an 07 I believe and had nothing but problems with it. But I believe that to be caused by the previous owner not taking care of it at all. Which is why I told him not to buy it in the first place. Which brings me to another point...
Things I would look at on these other than the obvious rust (especially being in Iowa), is caked on mud underneath and other signs of neglect. The front ends also have quite a few issues. So be sure to check that out, or just plan on replacing all the bushings, tie rod ends, ball joints, etc. after you buy it. I just did that to mine and it made a huge difference in ride quality. I believe it cost me around $1K including installation and parts.
I found mine last year with 170k miles on it for $4K which was a decent deal at the time. I wasn't wanting buy one with such high miles, but I figured it'd be worth throwing some money at to fix any issues. Especially since finding one with low miles was becoming very difficult if I didn't want to spend over $10k. So I guess my advise to you would be to find one fairly cheap, not worry about mileage too much and just plan to throw money at it to make it 100%. That will also give the benefit of knowing what's been done and what may need more attention. I haven't gotten into the engine too much yet (or the tranny), but I plan on replacing it with a remanufactured one within the next year or so which will cost me about $3k installed at a place down in Tulsa. After all is said and done, I'll probably have about $10k into my truck. Which I don't think is too bad considering everything will be reconditioned to new (or better than new) and I won't have to worry about anything for a long time. As opposed to spending $20k on a "new" truck with over 100K miles on it like most people do.
I'm not sure if I added anything to this post or not, or gave you any new info relevant to you. But hopefully I did.