Sounds like you are looking at the same truck I have. Mine is worth that much to me, but I wouldn't expect to sell it for that or pay that much for another one. I got rid of the stock 20's and put new rims and tires on, but other than that, mine is an 04 SLT QC Hemi 4x4 with sport trim (color matched bumpers). Mine has about 10k more miles on it.
Mileage seems to vary quite a bit for all of us here. I get around 14 city, between 18-19 highway.
Get specifics on this "chip" if you can. Is the truck currently programmed and not stock and, if so, is the programmer included in the deal? If not, without the programmer, you won't be able to change anything unless you get the ECU reflashed to stock (for a fee) and start over with a new programmer. I was never able to get a super chips programmer to work on mine because it said my settings were not factory stock even though I bought it new and never had it modified.
Service history and what has and has not been done to the truck, for me, would determine a lot towards the truck's value. If you buy one for 11k and have to put a grand into it because things need to be done or you have no way of knowing if they were done, is no different than paying 12k for one that is completely up to date on maintenance. Tires are also expensive, especially 20's, so if the tires are in good shape, that's a plus. If not, it should be accounted for in the sales price.
The one "bug" in some of the Rams is in the rear end. If the truck has a limited slip/anti-spin differential, there is a c-clip in the rear end that is known to fail and cause catastrophic failure. If it has this, there should be a label in the glove box that would specify something like 9.25 corp anti-spin. This can be avoided but of course that is an expense. I found that mine does not have the LSD rear end.
Other than that - very solid truck. Other than regular maintenance, I have replaced the EGR valve and a wiper washer fluid pump due to failure. Everything else is original and still going strong.