We got external insulation done on our 1980's semi-detached house in 2010, upgraded the windows at the same time to triple glazed and boosted the attic insulation. I was just having a look at our gas usage figures last night. Our annual consumption average since then is only 53% of the previous average. The payback time is long though, I was working out that we saved about €4300 in gas in the meantime, based on today's prices. I don't know exactly what we paid for the job at the time, it was done in conjunction with other works, but definitely not in the black yet. However the big improvement is in comfort in the house, gone are the cold internal draughts that drop down on you when the air within a room cools and circulates.
We didn't need to get anything done with the gutters, there was enough room under the facias to accommodate the insulation. We left the windows in the same position when they were replaced. We looked at moving them out as suggested above, but it deepens the interior window-sill and requires re-decoration, which is especially problematic in rooms like a bathroom where there was tiling around the window. We haven't had any issue with dampness or mould since. The windows were replaced before the insulation was installed and maybe that is significant in terms of avoiding cold bridges, and also the external reveals were insulated as part of the job.
Our front facade was a mixture of brick at ground level and plasterwork above. There was a cavity behind the brick and we got pumped insulation in it. For the remainder of those walls we got internal insulation. The main thing I think, is to consider you house holistically. It's like a bucket of water; if you ignore a hole anywhere, the water/ heat will leak out, irrespective of how well it's protected elsewhere. I'm sure the materials and technology has improved since our job was done. The competence of the contractors is a critical factor too, hopefully most of them know what they are doing by now. Good luck with it!