It really will depend upon your money and skill on whether you want to tackle this job. The benefits of doing this is once done you know how your home is put together and what quality of materials where used.
Many items can be replaced with items from Habitat for Humanity resale stores, Lowe's etc...
There are also alot of bargain stores and bins available.
Alot of times floor coverings can be bought in the discontinued areas of stores and fit. With creative thought and installs about anything can be achieved.
If you are financially secure or young enough to withstand a mortgage for one with same features or slightly better then yes buy new or upgrade.
But like in my case I bought one that practically every room had been redone by the previous owners and it was maintaining it and upgrading things to my liking that has justified near to 15,000 into it.
At normal labor costs it would have not made sense to do even the portion that I did. How much would it cost you to move into a home in better shape? You might want to chat with your insurance agent about how much they think it is worth and how long they will insure it.
BUT, you will serve yourself better if you post all heat and air questions in the HVAC forums.
When I started to look for a place, I found that the places built before 1976 (HUD regulation) were unregulated and often very shoddily built.
That would figure into any decisions I made, and in fact it caused me not to buy several places in more desirable locations. My ideology is how many bedrooms how many bath and what would it cost to build a stick built or buy a modular with those same features.
Underbelly, same reply as above and can be a PITA to replace but can be done.
My best advice is to get a inspector in there and check it out.Those had to be shaped to the shape of the ceiling. In this case it was worth doing because the house was in good shape and looks like the previous owners took good care of it.