1--Look again, there are many patterns for hats knit flat (some designers like Nicky Epstein almost never knit in the round!)
2--Knitting is knitting, and you'll soon learn that the main difference between knitting with straight needles (vs circular or dpn) is the needles, not the knitting.
3--consider a set of interchangeable needles
these can be used as flexible straights, or as circ, or even as stitch holders.
and as for which set to buy? it doesn't matter! every set has its lovers, and most sets can be resold for almost as much as you paid for them. (so if you find you don't like the denise's, sell them and buy a boye set, don't like those? try a harmony set.. don't like those? try a crystal palace bamboo set..
there are several brands/styles..(price ranges!) for sets.
You'll recover about 90% of the cost--(think of the 10% as a rental fee!) if you resell them--
NO matter which set you buy --all interchangeable sets are cheaper than buying needles individually--and represent a good value.
4--there are several tutorials for changing in the round pattens into knit flat patterns
add 2 stitches (selvage) for the seam
and use patterns that have "Knit every stitch on alternate rows" designs.
--to knit flat, the alternate rows become "purl every stitch, alternate rows"