Both engines in the frontier are good with little problems but I would stick with 2010/11 or newer because they didn’t have some of the concerns the earlier versions did 

Don't buy older than a 2010.  You can search my posts elsewhere, but what you need to check are the radiator for signs of SMOD (look under the cap....if it looks like a frothy strawberry milkshake, turn around and leave) and the differential breather to be sure it hasn't corroded shut.  If those things look good, you should be fine.  

I posted a list elsewhere, but stealing this one from a Frontier forum.  Check the radiator part number on the one you're looking at against these:

21410-EA200 [seldom referenced, but may have been installed in some very early 2005’s.]

21410-EA205 [2005 – either 2/09 or 8/09. Listed on some parts websites as P/N 21460-EA205. There is some confusion as to when these were used. Some literature states until 2/09 and other literature states 8/09. P/N IS ASSOCIATED WITH SMOD. ]

21460-EA215 [2005 - either 2/09 until 8/09, see above. Listed as discontinued and replaced by 21460-9CAOE on Nissan parts websites. P/N IS ASSOCIATED WITH SMOD]

21460-EA265 [production dates unknown. P/N IS ASSOCIATED WITH SMOD.]

21460-ZL11A [8/09 through 2010 model year, isolated reports of SMOD.]

21460-ZL31A [production dates unknown. Listed as discontinued on Nissan parts websites , no known reports of SMOD.]

21460-9CAOE [2011 – 2015 model years – no known reports of SMOD] This is the part number for the most current version as of 11/16.

I would consider a four cylinder if you don’t plan to tow. 

BTW, there's no reason to stay away from either motor.  Both are quite reliable, but the V6 is noticeably more powerful.