Probably dumb questions.

I recently bought an xterra and I have a few concerns. It's a 2006 4wd. I bought the x from a local small dealer, it was as is with no warranty, and of course I got the "runs perfectly" speech. The day after I bought it it started to overheat and lost a lot of coolant, so I had it checked out, they said it was an air bubble in the heater core so they purged it. A few days later it began getting hot again, so I took it somewhere else and they changed the thermostat , so far it's working well however it still reads hotter than I noticed in my old car, it gets just above the temp icon. Is this a normal running temp. Also, I can hear what sounds like coolant moving through the system. It isn't boiling, I heard the bowling before I changed the thermostat, it sound more like gurgling when stopped and when I turn it off. It also sounds like the coolant is rushing through the system when I accelerate sometimes. Another thing I've noticed is that when I am going downhill or not actively using the accelerator the rpms go up, almost like its shifting. There is also a small leak of what I think is oil, it's a very small and slow drip. Lastly, ok not sure if this is even a real issue but the engine sounds "Whiney?" Almost like the sound that's made when the power steering is low, however it does r make the sound just when I'm turning, it also gets louder (maybe a little higher pitch) when accelerating. One more thing, sorry, when the slip control kicks in, there is an unsettling grinding noise. I don't know if any of this is related or if its even an issue. I'm just very paranoid, also I'm broke.

Sounds like what you bought is worn out. If it is not too late, I would take it back and tell them to fix it - for free. They should not have offered a vehicle with so many problems - unless they sold it cheap and told you something along those lines. "As is" is a common way of presenting a used vehicle, but the reputation of the seller is at stake when problems like this arise. Give them a chance to do the right thing. You may need to get in their face, but only as a last resort. What you have described may cost you thousands of dollars to repair.

Great, do you think I should have a mechanic look at before I go back to the dealer? Would it be better to have a nissan dealer check it out?

It may be a good idea to take it to the Nissan dealer. I would call ahead to see if they have a free 27 point inspection (or something like it). It is a good time to establish a relationship with them.