This is what I do when looking at TV's.
Generally I only look at Plasma's and not LCD due to sports and movies generally looking better on Plasma's, although LCD panels are better now to me they just still cannot give as good a image as Plasma's.
I generally only look at 2 brands which are Panasonic and Pioneer, as they generally are better quality from my experience off selling and using TV's as they had, a lower fault rate than the likes of Samsung, Sony, Toshiba and others when it came to people coming, back to the store for either replacements or to book a engineer.
I look at whether they have a "Game" mode on them, as that dependant on how it's implemented can help reduce the input lag that most tv's suffer from, but again I have been out of the loop on the latest set's so they may not suffer as bad from it any more.
I look at the input and output options to make sure it has enough connections on it for everything from a cable box to audio out.
After all that I look at the warranty side of it and see if I can see it in a store first to see what the image and sound are like.
As for the size see if you can get some cardboard and cut out the dimensions of the tv in cardboard, and sellotape it to the wall where you want the TV to go and then sit, where you normally will when you watch it or play games.
Remember the screen inch size is from corner to corner of the panel from inside the bezel and not from the corner of the tv to the other corner including the bezel, as most people see a size of a tv in a store and then when they get it home it looks much bigger than it did in the store.
That is generally what I do, I don't care about all this "Smart TV" stuff because I have a media pc connected to my TV, so I don't need to use the built in media player apps although they are pretty poor on my Sony TV, as they struggle with 3D films and some other films because the Bit-Rate is high it's as if the TV is just not powerful enough for it.
If you decide on a 4K TV make sure it can actually do 60Hz unlike some of these supposed "600hz or 120Hz" tv's that when you connect a pc to, them they will only display at either 30Hz or 60Hz, and make sure it has the right connections to support 4K @ 60Hz otherwise it's money wasted IMHO.
***Edit***
Just to add I have a Panasonic 42" 1080p TV that I have had for over 6 years now that is working perfectly fine still, unlike the Samsung that was bought 3 years ago and the Sony that I bought 12 months ago which is starting to act up a little.
