Since I used to work at a dealer ship I'll say. Trans take it to a good trans shop and have them change ''all'' the fluid in the trans and ask them to use the thick Chrysler gasket .. This stinking gasket cost about $40.00 but about 2 weeks later you'll be glad you used it. After you get over the trans job. I'll say you'll spend about 150 on this. If it still slips then you need a rebuild .
... $600.00 I'd say you stole it but you have to bring it up to date. I got mine for $400.00 and say items you said. New battery .. New fuel ...tune up. I had to have the rebuild on my trans $1000.00 .. A Set of new tires ..
.... Mine still ran rough So I ran 3 tanks of 93 gas and about as many bottles of Lucus thru it and I've had it on about 4 road trips.. I'm amazed at how much better it is running .
..... Good Luck.