No QFT isn't needed to understand inflation. A good set of math skills and thermodynamics understanding is sufficient. Though GR does help along with QM.
The common formula used in inflation is the equations of state for scalar modelling. This is your thermodynamic portion. One major aspect in both GR and cosmology is both the FLRW metric and Einstein field equations employ the ideal gas laws.
The scalar modelling EOS can be found on this link.
https://en.m.wikipedia.org/wiki/Equation_of_state_(cosmology)
As mentioned before one needs to be able to employ the math, that math relies on differential geometry , strong statistical mechanics is also a good asset.
This site has 3 good preliminary textbooks.
http://www.feynmanlectures.caltech.edu/
Now as far as QFT itself is concerned, it's good to have some particle physics understanding in regards to inflation and Early universe dynamics. However the necessary details can be learned through either Quarks and Leptons
https://www.amazon.com/dp/0471887412/?tag=pfamazon01-20
or Griffiths Introductory to particle physics.
http://www.amazon.com/dp/3527406018/?tag=pfamazon01-20.
Here is two free resources. One being a full length free textbook.
http://arxiv.org/pdf/hep-th/0503203.pdf "Particle Physics and Inflationary Cosmology" by Andrei Linde
http://www.wiese.itp.unibe.ch/lectures/universe.pdf:" Particle Physics of the Early universe" by Uwe-Jens Wiese Thermodynamics, Big bang Nucleosynthesis
For the needed math in particle physics a good free resource is
http://arxiv.org/abs/0810.3328 A Simple Introduction to Particle Physics
http://arxiv.org/abs/0908.1395 part 2
The first book is primarily a coverage of differential geometry. 90+% of it, the second book steps you into GR.
GUT theories, are a bit trickier, in the coupling constants and is largely model dependant, (standard model vs SUSY and SO(10) , both MSM- minimial standard model or Minimal super symmetric ).
It's handy to be familiar with the basis of GUT but not necessarily required. Unless you wish to develop your own models.
Here is a decent coverage, the previous references will greatly help to understand this article.
http://pdg.lbl.gov/2011/reviews/rpp2011-rev-guts.pdf GRAND UNIFIED THEORIES
Two excellent intro level cosmology textbooks is Barbers Rydens Introductory to Cosmology.
http://www.amazon.com/dp/0805389121/?tag=pfamazon01-20
and
Introduction to Cosmology by Matt Roose.
https://www.amazon.com/dp/047084910X/?tag=pfamazon01-20
Both are excellent entry low math intensive textbooks.
Modern Cosmology by Scott Dodelson
Requires greater math skills
https://www.amazon.com/dp/0122191412/?tag=pfamazon01-20
As well as Physical Foundations of Cosmology by Muchanov.
http://www.amazon.com/dp/0521563984/?tag=pfamazon01-20