Taking things in reverse order:
With the large areas firstly remove any remaining loose plaster. Trying to patch without doing this is a waste of time. Then with the large areas you'll need to replaster. You can either take this as an opportunity to learn or pay someone. I won't tell you how to plaster as it's really a skill you have to be shown or practise yourself, though I do have a couple of bits of advice. Don't try to plaster the whole wall in one go. Take it in stages and make sure you dampen the wall you're plastering onto as it helps the plaster stick.
With the areas missing the top coat of plaster you will need to reskim. This should be done after fixing the base coat. You might find a plasterer willing to skim over your base coat - discuss it before you start. This will give you a good finish but should cost less than hiring a plasterer to do the whole job. Getting a good skim finish takes a lot of practice.
Finally the areas with only minor cracks and bumps should be filled and sanded before proceeding.
If you manage to get a good surface then painting is the best option. If not then paper the wall with two layers of lining paper. The first runs horizontal and the second vertical. This will smooth out most of the remaining unevenness in the wall. Before papering wash the walls with a weak glue solution this will stop all the glue from the paper being absorbed into the plaster and the paper lifting off the wall.
I should have added that you need to leave the plaster to fully dry out before painting or papering. That might have been one of the reasons why the previous repair has failed.
UPDATE
The quote you have doesn't seem unreasonable but without knowing the exact area you need replastering or your location it's difficult to say for certain. However, we shouldn't be getting into commenting on exact quotes as they are always going to be too localised (in time and space). Get another quote to compare it against - that's always going to be the best option. Also as I said above see if you can get the plasterer just to do the final skim - it should come out cheaper.
As Tester mentions, new windows are the most cost and energy efficient in the long haul. However, the upfront cost is substantial, especially if you want to do a lot of windows.
The concept is the same with all window coverings, add an additional layer of air between the primary window and the covering to slow down the transfer of heat. A single pane window is generally .85 r-value. Verifiable r-value numbers are hard to come by for most of these products, but if you do find some, that will give you a basis for comparison.
Other options:
Storm windows. This is basically installing a secondary window on the outside. These can be quite expensive, or cheaply made using plexiglass. They are cheaper than new windows, reusable, and transparent. The drawback is that there is still a sizable upfront cost and you have to store them somewhere in the summer.
Secondary glazing. Same idea, except installed inside. Similar advantages and disadvantages to storm windows. They tend to be much thinner and don't change the look of your house.
Cellular shade. There are some interesting claims with cellular (honeycomb) shades, some claiming to add well over 2 r-value to a window. These will darken the room some and are still quite expensive.
Window film. This is the cheap option, unless you have curtains laying around. The main drawback is that it's generally opaque and it's more difficult to reuse year-to-year.
Heavy curtains. Tried and true method used for centuries. Obviously you won't be looking out these windows.
Best Answer
Strip off all the old paint and sand the wood very well. Then use a good primer that seals the wood that water cannot enter it. Finally paint the wood in a colour of choice.