Ab oil flush is good idea too but it will be money wasted if she is "mixing drinks".
I have used this one over many years. It works very well. I got caught by it once with a very tired old turbo diesel - it was so clagged up that when I flushed with CEM the thing blew white smoke profusely and could no longer be driven on the road as all the old muck was gone that kept the rings sealed LOL My fault for buying a 4WD off a lying scumbag...
https://costeffective.com.au/product/flushing-oil-concentrate/
They recommend you put it in with clean oil and that it will need the clea oil to absorb the muck and they are right. I have had engine oil turn into black sludge in twenty minutes with this product!
Try some chemiweld or such product to stop the coolant leak. If it saves the old engine for even just six months you will be better off than some mega expensive rebuild on a n old car you just bought. Best to just drive it a while and see what else is wrong before rebuilding an engine then finding you wasted all your money when so many other probs crop up. Good luck with getting the old girl ship-shape. Old cars save the planet!