There were a few wooden storms and screens left on my house when I bought it. They're gone now, but they were some sort of western softwood (cedar?) and had full slip joints in the corners. These storms aren't worth much if they don't fit well, and it's likely that your old windows will be something other than square after 80 years or so, with layers of uneven paint and caulk making them difficult to fit anything to. Doing the job well enough to be worth having done it would not be all that simple, and it doesn't sound as if your shop is set up for it.

"1. Type of wood to use??? I'm going to paint them. Poplar??"

Poplar accepts paint well but is a disaster on exterior exposures.
Pine actually works better here. Cypress or redwood wood be a step up from pine with the preference probably going to cypress.

"2. Jointing? The simpler the frame is the better for me, but i'm not sure what would work best for seasonal windows."

Half laps will be more than adequate. Use Titebond III glue.
Wood movement is not usually much of a problem since the cross sections are normally less than 3-4 inches.
You can get various types of weather stripping to make up for variations in the old windows and surfaces.
Resources Conservation Technology has many styles available.
Some for edges, some that go in grooves, etc.

There are multiple ways to make the frames up and cut the required rabbets for the glass.
A relatively low tech method is to build the frames and then use a router to cut the rabbet.
Some hand work will be required to square off the corners.
Prime, install glass (push points are a simple way to hold the glass before putty), putty, allow to set a few weeks, then paint.

Could I just cut a dado slot down the side of each board and slide the glass into the slot instead of cutting a rabbet and glazing the glass?

just a thought, like one person saiid, pine is a very good exterior wood, poplar is not only very suseptible to expanding contracting in the weather. it can be painted but staining also has some weird results sometimes (just fyi) a good exterior paint will do well as far sealing the wood and preventing rot and water damage (agian just fyi), the other thing i wanted to mention (as has been mentioned) is that this is a particularly tough job for someone not experienced in this field, if it was me (as ive never made a window frame either) I would have someone whose more experienced take a look at it and see what youre doing right/wrong

I have repaced the old screens on my 1929 built home with shop made storm windows. 5/16 rabbit 45 deg cuts on top with biscuit joint and rabbited square joints on the bottom. Used number 2 pine and painted, applied weather strip and they work and look great. When I first started made one a day. got easier as I proceeded and made 3 a day. I used my home made router table to cut the rabbits.