2002 Buick Century overheated this summer, replaced thermostat, problem solved, until yesterday- overheated in cold weather, what could this be?

We bought our 2002 Buick Century from a mechanic at his service station one year ago- to the day. Since the beginning, there were problems. My husband is very sick so he wasn't thinking straight when we bought the car. In the beginning, we heard a noise which my husband said is the "lifters", and that hasn't really been a problem other than the noise, which goes away two minutes into starting car.
Then came the wheel bearing, which we had to have replaced- $330!
Then came the overheating problems. During the summer, the car would overheat now and then, no rhyme or reason to it- and finally it kept doing it so we had no choice but to get the thermostat replaced and pray....well, it seemed to work cuz until now, (about 4 months plus) it never overheated again.
The first 2 blower speeds for the heat/A/C never worked. But in the past week, oddly enough, the 3rd blower speed went- and now we only had the 4th to depend on.
We were in the middle of researching THAT problem, thinking perhaps a blower motor resistor- blower motor problem- when the car overheated....twice.
The gauge went up into the red- turned on the highest blower level, NO HEAT now at all, but the gauge did go down below 12 oclock.
We are so angry at this dam guy who sold us the car- 4 grand for a pile of problems.
We have one income- husband is on the transplant list (UNOS) five years- we have multiple appointments, med refills, etc every single month- in and out of hospital- and now, no car, and no savings to fix car.
ANY ADVICE IS GOOD ADVICE- we could come up with a couple hundred dollars by borrowing from those we love- if we had to- but that is about it.
As my husband puts it, the car is an "electrical nightmare"- and had he not been so sick with what is called "Hepatic Encelphalopathy" (toxins in the brain making you sometimes in a type of stupor-not yourself at all) we would NEVER have bought this car. I can PHYSICALLY do the things he can't to the  car- we work as a TEAM, so any troubleshooting we CAN do= he has a lot of tools- he has always worked on our cars and always come through shining like the sun- he just cannot get into the positions needed to fix the car-sorry so long, just wanted you to know (hey, everyone has a sob story, right? LOL) so maybe you could get back to us on this- just some feedback, advice, trouble shooting ideas- thanks so much!!!!
Boston, MA