Help! Car's in the shop right now!!
If I were your daughter, what would be your advice (
other than you probably would have bought me a better car! LOL)

in the following situation: Read the whole thing, though, because it's 3 different days/issues)
!. Just bought used
2002 Taurus ses - 134k miles - turned on AC while stopped at light & the car cut off but started back up. But then it cuts off while idling too though, but not every time. Also "Low coolant" light is on but fluid level is actually okay. Today I was "flooring" it while getting on the expressway and it wouldn't go faster than 50 - has no pickup. It eventually sped up after I let my foot up off gas and it sounded like it "changed" gears. Has no pickup. Mostly, it is "drinking" gas!
2. I took my car to my mechanic who says I have a "bad vacuum leak" and it's causing my troubles - the lack of pick up, cutting off, drinking gas, running rough - once the hose is replaced and he checks for any other tiny leaks (by running some dye or something through it???) I should be good to go.
3. NOT! After I picked it up, it was okay for about 30 minutes! Then it started making a horrible noise and was letting off some awful fumes - I took it right back! Now they say it's the catalytic converter - They said not only were there tiny vacuum leaks but the ETR (?) is melting - they tried to replace it and it was so hot that it started melting that one. So now they say the "emissions tube" may be severely clogged and it would be best to replace the catalytic converter. (To the tune of about $500!)

Does this sound right or logical? I looked up catalytic converters on the "How stuff Works" website, as well as a few prices - they seem to run from $230 to $476 online. (Before they order one - if that's what they're gonna do - I want the best price!)
The best I can tell is - shouldn't they check to see if it's clogged? Can't they just unclog it? I realize it's underneath the car and somewhere between the engine and the muffler on the passenger side - if some tube or hose is getting so hot that it's melting stuff under the hood, why don't they just replace the part that's clogged?
I am trying to learn all I can so I am not taken advantage of! Please help! Please "school me"