Try a paint store. they are usually helpful on such subjects, and I don't mean the paint DEPARTMENT at the local box store, I mean a name brand or otherwise independent paint store.
#1 nylon or synthetic bristles for acrylic or latex finish coats.
#2 don't make the brush too wide. A house paint brush is usually 4" but if you're not used to that kind of work you're in for a sore hand.
#3 angle or straight is personal preference, but most exterior brushes are available in both, up to a certain width.

I posted here because these guys are BETTER than a paint store. Read around and you will see. Plus I had an unfortunate experience at my local paint store. Here is better no doubt. Thanks for your suggestions.

I have two favorite brushes:

A Wooster Shortcut - my first "real" brush, still a fave because of the way it feels in my hand, and it doesn't do a bad job of carrying paint

And my splurge: Purdy Chinex (I think a 2.5" "Bow" style, straight cut not angled). The Chinex brush holds a lot of paint, smoothes it on beautifully, and cleans up like a dream (so you don't have to stress over paying $20-30 for a brush you can't get clean)

My other go-to brush is a 2" Purdy Dale, angled and doesn't hold as much paint as the Chinex Box, so better for details and cutting in, IMO. Used to use the Shortcut for that, but I abused it and use it mostly for rough stuff now. Live & learn.

A good brush holds a lot of paint so you can make longer strokes and not waste time constantly re-dipping, doesn't drop bristles like a cheap brush, and gives a smooth finish with minimal brush strokes.

Seriously, if you've been using crappy foam brushes, try using even an $8 brush and see the difference. Having a decent brush is truly a time saver, plus it makes painting so much more pleasurable!

I'm headed to bed, so that's the short version. I'm sure paintguy, christophern, faron or graywings will be by to elaborate on types of bristles, etc.

Depends on the paint.

I have Duration, I forgot what the sheen is, not flat but a very light sheen.

You should realy use a small 4" roller and then back brush the paint after it is on the wood. Just using a brush will take forever and is hard to do that and end up with decent results because the paint is drying too fast while you are going to the can to get more paint. Also, don't paint while the sun is shining on that side of the house. Wooster, Corona and Purdy all make good brushes. Usually this is personal preference. Angle or straight is personal preference. I use 2.5" brushes for 95 percent of the painting I do. You will not need a foam brush for anything for sure and stay away from the cheap packs of brushes too.

I hardly ever use anything but a long-handled 2.5" straight tipped brush. The short handled brushes end up giving me hand cramps.
A chisel-tipped 1.5" brush is great for cutting in against glass.
Wider brushes are too heavy with paint, and lead me to hand fatigue.
For trim, I roll on paint and rapidly brush it out.
Obviously, use a bristle that's appropriate for the paint formulation. Black china bristle (oil) is for light-color paints, white bristles for dark colors, so you know when the brush is clean!
Don't curse a new brush for dropping bristles, they need breaking in. Put them back in the wrapper after cleaning.
Casey

I went to Home Depot today to get my brush. They did not seem to have any 2.5 real bristle brushes made by Purdy on display so I got something called a Purdy 'Nylox'. Seems OK, was not cheap.

You did not want a "real" bristle brush anyway, they are for oil.