First off, is it 300K miles or kilometers? If it's miles, that engine is probably pretty tired. I wouldn't dump a bunch of money into it. If it's 300K km, that's not quite as bad. If the car was driven any length on time, it (engine) could also be toast. A friend of mine tried to drive his home and cracked the head AND the block. New engine, big $$$.
Is it blowing coolant out the tailpipe really bad? Is there oil in the coolant or coolant in the oil? If the amswer is no, you may still be in business. You have a couple choices IMO.
You can pay somone (5 hrs) to tear it apart and find out how bad the head is and if the block is cracked, (hard to do though). Then, you'll know where you stand and can make an informed decision.
Or, you can do what I just did and with your miles it might be a good alternative. I bought a bottle of CarGo Seal Up at Pep Boys. I was very skeptical about this, but did some research that indicated this stuff might actually work.
It worked for me, so far at least. I still get a small amount of coolant into #3 cyl after sitting overnight, but you wouldn't know it by the way it starts and runs. I'd say, it's 98% healed. Huge difference. It's supposed to be a permanent fix. But, I'm just trying to buy some time until Ican afford a proper fix.
The key is to follow the directions EXACTLY as stated on the bottle. Making sure you get ALL the coolant out. I also let it idle for twice the time they suggest and let it air dry three times longer. Best of all, it only costs $4 a bottle. BTW, don't let the cheap price fool you the govt. tested it and others like it on cooling systems for neuclear reactors. The CarGo Seal Up was the ONLY one that worked.
If your engine has 300K miles on it, it can't hurt. You'll only be out $4.