Im wondering what kind of tv to look for?
IMO 4k is useless because the network where I live is bad and 1080p is good enough. I don’t think smart tv functionality is a thing to look for, more like a « what not to look for » criteria.
What other criterions should be taken?
The cheapest one in the size you wanted. You can get 55" 4k tvs for as little as $250 when on sale at microcenter and they work/look great. Buy a $50 sound bar on Amazon as all TVs in the low range have terrible sound.