There are a lot of unanswered questions here. My suggestion would be to measure your windows and talk to someone at the store where you plan to buy them. If your windows are not standard you will be paying more for them as they will have to be custom fitted. I'm guessing the most important measurement would be the thickness of your walls. Most newer models have pretty standard stick built specs. Older models have thinner walls and make finding ready built windows difficult to fit. The same is true of doors. If you're not replacing the door itself then the thinner walls make a difference and hardware would have to be special ordered through your dealer.

Insulation ---- The walls and ceilings should already be insulated. If you're replacing insulation then that becomes a major job as the old needs to be removed first. If you remove siding to remove insulation then replacing it is no problem. If, for some reason, there is no insulation at all then you could look into blown insulation but I don't think you could blow insulation in on top of old insulation.

We ended having to custom order all our new windows and used the "New Construction" type, these have lips on the outside of them, easy to put rope caulking around just before installing and they screwed right into the side of the house without looking horrible. It was either this way or getting standard windows and cutting out and forming new window frames or trying to match siding / replace all the siding on the Mobile Home. We went with the less expensive way by just ordering the sizes we needed. We also watched our local store ads and found that they even have sales on custom orders every so often (major National Chain Home Improvement Stores) If you have extra room on the inside frames when you insert your new windows, just by some nice 1 x ?" pine, cut to fit, then paint or stain the color you need. Turned out really nice and OH THE SAVINGS ON THE UTILITY BILLS paid for the windows in two years. Best of Luck