Welcome to the forum - Looks like you are a long time lurker, 1st time poster.
This is the age old question everyone has to answer for themselves when they decide to fix an older car or not.
As has already been asked, are these things you can do yourself or need to take to a shop to get addressed?
Also, I am going to have to call BS on the fact that they "all hit at once."
As for the tires, these should not have been a surprise as they didn't just suddenly wear out. For whatever reason, you just decided not to do anything about them until now and winter is coming. The need to save money for new tires will not go away with a different vehicle. That being said, I am guessing you will not be installing the tires yourself so tires will run you around $800 depending on what tires you go with. This is a safety item so should be a priority.
The oil leak, unless it is just gushing out all of a sudden, probably didn't just happen and has been going on for a while, and again, you made the decision, for whatever reason, not to do anything about it until now. That being said, it can probably be left to go a little longer as long as you keep the oil topped off and don't mind some oil spots under where you park. The fact that you have no idea where the oil is even leaking from tells me that you have been ignoring it up until now.
Rear end work is another area that most folks don't delve into themselves so I would plan on either spending $800 for a good used one or roughly the same amount for someone to rebuilt yours.
Windshield will probably run you around $500, for budget sake and again, and is probably not something you will do yourself. I am assuming that this is not an immediate need and may be something you can put off a little bit longer unless the windshield is caved in for some reason rendering the vehicle unsafe to drive.
As for the heater core, that isn't that expensive of a part, just a pain in the rear to get to so if you can do this or find a friend that will help for a case of beer and some pizza, that can be done without a huge layout of cash. If you take to a shop, you will pay handsomely for this to get taken care of. Same goes for typical causes of oil leaks.
If you do the work on the heater core and oil leak yourself, you are looking at around $2000 in parts and labor for tires, windshield and rear end work and then $200 in parts and fluids for the heater core and most common causes of oil leaks that folks will deal with. You need to make the call on fixing your current vehicle or spending much more that $3000 on a different vehicle that will come with its own set of issues that will cost money to fix.
Sorry if I sound a little stern, but when people ask whether they should fix their vehicle or buy a different one and appear to not have taken the initiative to maintain the one they currently have or plan accordingly for known wear items like tires, I will never suggest getting a different one because the same thing will happen. Things will get ignored, problems will get kicked down the road and then eventually they are right back at the same place saying their vehicle is a money pit and I need to get another one.
