I can't speak from personal experience, but I did some research on porch enclosures for a house we're looking to build next year that you might find helpful.

We're currently looking at a system made by Harvey Industries http://www.harveybp.com/product.aspx?pid=8&shape=14 which is extruded aluminum. These are purpose built, pricing starts around $60/foot of enclosure.

Others:

1: Frame porch to allow wood screen door panels to be inserted, see http://www.thisoldhouse.com/toh/article/0,,1541906,00.html for a description. Lots of door choices ranging from wood to vinyl. There are purpose built mounting kits http://www.screentight.com/prod-screen-doors-vinyl-doorpanelenclosure.shtml and acrylic panel kits http://www.screentight.com/prod-screen-doors-vinyl-windbreaker.shtml available for certain doors.

2: Removable Aluminum/Fiberglass Porch Panels - see http://www.qualitywindowscreen.com/store/patio-porch-screen-systems-patio-porch-screen-panels-c-101_176.html for an example. Not sure about glass on these.

3: Screentite and similar. See http://www.screentight.com/prod-screen-tight.shtml or this one which allows windows: http://www.kaufmannwindow.com/porch.html.

4: Inexpensive vinyl sliding glass doors. See http://www.thehouseofwindows.com/Sliding_Glass_Patio_Doors_p/snc5068.htm for what would likely be a true 3-4 season porch but expensive.

Thanks for sharing your research, crdnh. Are you in New Hampshire? We're in Massachusetts, and have been pretty happy with other products from Harvey Industries. They're not high-end, but they're reasonably priced and offer good service and warranty.

I didn't know Harvey had a porch enclosure system, so thanks again for sharing that info. Given our previous good experience, I think we'd be happy to go that route.