Hi Tr0LL,
Wow … that’s an old Tempurpedic and you don’t see many of them that have reached that age!
If you decide to go in this direction then the Dreamfoam 13" mattress certainly a great quality/value choice but it is also much softer than yours and would be similar to the current Tempurpedic Cloud Luxe so you could test this in the store to see if it’s a good match for you.
If you had bed bugs you would know it (they bite)!
If it was me I would be leaning towards replacing a 17 year old Tempurpedic and if the foam is ripping or cracking then it’s probably a sign that it’s breaking down but if you are still comfortable sleeping on it then why add a topper at all? The most effective use for a topper is to add additional softness and pressure relief to a mattress and if you are used to sleeping on a firmer mattress then the risk of adding a topper (or buying a softer mattress) is that you may end up with a sleeping system that is too soft for you and doesn’t keep you in good alignment.
I personally would be cautious with memory foam that was less than 4 lbs in terms of durability but with a topper then going as low as 3 lb density would be less risky than using it in a mattress both because of your lower weight and because you can replace it without having to replace the whole mattress. Having said that … lower density memory foams are often less “memory foam like” and faster responding than higher density memory foams so it would really depend on how you feel about the specific properties of the memory foam you are purchasing and on how suitable the mattress/topper combination is for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences). There is more about the different properties of different types of memory foams in post #9 here and post #8 here.
Phoenix