I have done almost exactly what you are describing. I restored a little house that I lived in as a child in ND. Have lived in CA for about 50 years and did the restoration in ND a couple of years ago. The people that moved in the little house after us took all of the windows out of the porch and actually used some of the porch space for their bedroom. I wanted everything to "look" like it did when I lived there. I put expensive Marvin windows in the rest of the house, but wanted to save money on the 12 windows in the porch. My carpenter first put in just the storm windows that come from Marvin.. What a mistake that was! Actually had snow in the window sills. The next summer we took out the storm windows and replaced them with dual glazed vinyl windows made by Summitt. They were not expensive. I think the reason that they look good is because of the wood trim that is used around them, both inside and outside. I would like to learn how to post some pictures because I have some that clearly show them... I visited ND last week, nearly froze, but used a really cute heater that looks like a fireplace in the front porch and it stays toasty warm in there. I do have forced air heat going to the porch, but it isn't quite enough for me without the electric heater.. I love to sit out there and just look at the snow and the empty land. I might learn to post photos and will do so if I can.

We enclosed the back porch of our house very inexpensively. We found a set of big wooden storm windows at a garage sale for just a little money. We framed in the storm windows and added a wooden screen door with a glass insert for winter. If you want to don the same, you might try advertising in a local paper.

If you aren't planning to insulate the rest of the structure so that it will be comfortable for winter, you don't need double glazing on the windows. Ours do frost up on really cold days, because the rest of the house leaks warm, moist air onto the cold porch.

About four years I added a three-season porch off the dining room of my 1910 Colonial house. It is about 16.5 by 12.5 feet. I used Pella casement windows, four on the long sides and two plus the door on the short end. There is a high ceiling with a fan and it says comfortable most of the summer because of the ventilation. There are baseboard electric heaters on the short side -- either side of the back door -- and it is warm enough to go out even on the coldest winter day. It is my favorite room in the house.