I have a base 99 Dakota and was wondering what years will interchange with the 99. I want to replace the rear bumper and maybe change from a bench seat to factory buckets,

 

Dodge is weird when it comes to interchangeable parts - It's almost maddening. The best luck you're going to have with your 99 is going with 1997-1999 Dakota parts. Again, not every single part will work but the 97 through 98's are going to be your best bet and only practical option.
The bumper shouldn't be too much trouble but the seats probably will, especially if you're gonna go with factory buckets.
The seats in my 98 were (and kinda still are) a mess; the foam is deteriorating, many of the springs that secured the support mesh were missing and the holes that the springs wet through in the seat frame had pulled through (I had to re-drill new holes in mine).
I'm not by any means saying that it is impossible but it will most-likely be a bit of a challenge.

Best of luck!

 

Do you think a 2000 Dakota's rear bumper will interchange with the 99? I found one of those.

 

The rear bumper should be the same. I know they changed the front bumpers a little but I dont think they did anything with the rear. I dont think the seats are gonna work from a new year since they changed the interior style. Plus the fact you are going from a bench to bucket seats. You can look into leather durango seats. Even those require a little bit of modification but they are very comfortable seats.