i'd be looking for deals on the classifieds section of http://www.turbododge.com
you can find a '782' swirlhead anywhere and i wouldn't pay more than $50 for one from a salvage yard.
there'* bound to be someone in your area on the forums with a complete engine or parts car with a 2.5 TI, and probaby for $500 or less. you may have to drive a little ways to go get it, and then just drive it back.
and yes your current transmission will work fine, although i'm thinking the A413 that went into turbo cars was proabably beefed up and probably had a different torque converter. really i would register on either of those forums and ask what needs to be done if you really think you might turbocharge it. i really think you'll find that it'* going to be EASY and CHEAP. it'* really all bolt in including the electronics for the most part. the 2.5 TI is not a hard to find engine.
i'd imagine that LeSabre would get horrible gas mileage.