Just 3 weeks ago bought an 01 RX300 with 23k miles, CPO from Lexus in Dallas.
Loaded, wood wheel, no nav. $27.9K
1. Your dealer will try to get retail plus about $1K for CPO. Research retail on Kelley or NADA and work down from there. The CPO is piece of mind for an additional 3 years or 100k miles total from the day you purchase. I feel $1k premium is worth it. When you buy from a Lex dealer, they will offer you the 1 time chance for 2 additional years of warranty (I believe this is 5 years from your date of purchase and 100k total miles). I declined this $1900 offer but assume this additional warranty coverage could also be negotiable. Work them hard, shop around, try waiting until the last days of the month when salespersons are anxious to sell, and remember that while the RX300 is a great vehicle, it is no longer a current model and we may see more depreciation in the coming years.
2. CarFax for sure to check for major probs. A Lexus service record is great to have. My dealer proved that they sold this vehicle to a local owner who had all service (including oil changes) done at that dealership. No major probs or warranty issues is nice to know. If Toyota/Lexus maintenance records are not available, they probably won't have proof of other maintenance. As you probably are aware, oil changes at least every 5k miles are important for the RX you are looking at.
3. Transmission and sludge appears to be major concerns. Be wary of repeated probs that may pop up on maintenance records. A CPO vehicle should look and drive like new. Don't accept anything less.
4. I would work on possibly negotiating the additional 2 year warranty coverage or perhaps include the 30k maintenance when you reach that mileage. If you don't get 2 master remote keys (plus valet and wallet key), you should push for a 2nd master remote key. I got my dealer to give me the 2nd master key and a new set of front/rear carpet floor mats. Anything is reasonable when negotiating the purchase of a vehicle. If you don't ask for it, you sure won't get it.
Good luck with your purchase!!