There are some on here that will tell you to run far far away from anything with a Northstar. Thats not me.
You cant really do anything to guarantee it wont happen again on a car that HASNT been timeserted yet *BUT* you CAN look for one that has ALL its maintnenance records from DAY 1. For a 2000 that would 1 if not 2 coolant changes already, all the oil receipts for oil changes according to schedule, all warranty repair documents and out of warranty repair documents, and prefferably from the original owner. I dont profess to know anything about your budget or financial situation... but if you must stick with a Northstar, id recommend at least a 2003 with lower mileage. We havent yet seen any 2003's with gasket problems here... that doesnt mean they are any less susceptible... but WE havent seen them yet if they are. Moreover, it will be easier to find an 03 - 05 thats got service history and less maintenance that would have needed to happen thus far, *PLUS* most likely withIN factory warranty.
Which brings me to my NEXT point... You can get another 2000 if you want to go that route, but get a warranty from a reliable provider like WarrantyDirect that sells exclusionary rather than inclusionary warranties. Meaning they tell you only what is *NOT* covered, rather than just what IS. If Seals and Gaskets arent in the NOT covered list... then they HAVE TO pay out if the Headgaskets go on the NEXT car. The newer the car, the cheaper the warranty. Either Certified from Cadillac, or with a warranty you purchase aftermarket... I wouldnt own another one without it if I had to purchase a higher priced one.
If you dont want another Northstar, which I can understand... maybe youd like a CTS or one of the older LT1 based Caddies? For me, there is no comparison to the Northstar models in sheer beauty and interior refinement... but that motor is a big deciding point.
Last, I hate to send you away... but Lincoln has several fine options too... If you liked the DTS, you may or may not like the 2003 Town Car or better yet, my favorite... the 2003 Lincoln LS V8 Premium Sport with THX Nav and Heated and Cooled seats. One of the BEST domestic luxury values on the road right now. Even the 1995-2002 Lincoln Continental would give you similar luxury to the DTS, though no where near the refinement of the Cadillac.
I know Im not being much help... but that will be your ultimate decision.