Hi ZzzCT.
Welcome to our Mattress Forum. 
Wow! 17 years, that is quite the lifespan from your mattress. You’re right that the memory foam is probably well passed its prime and needing replacement.
This really depends on your support needs. To steal some words from Phoenix… There are some comments about thickness and having more layers in post #14 here that should be helpful.
You certainly aren’t anywhere close to a weight range that would “need” more than 9" of latex to sleep well at night but depending on the specific design of the mattress and the layers inside it and where they are in the range between “princess and the pea” and “I can sleep on anything” there may be some people that would notice a difference and may “prefer” a thicker mattress.
This does seem to be an attractive option, given that you would have 2 additional layers to use should you need them and, as you pointed out, ease of returns and exchanges. However, Foamsweetfoam is very knowledgable and would be experts in helping guide you towards a great, new configuration.
But, if you’d like to do some additional looking around I’d say Arizona Premium and Sleep EZ (with their nifty mattress building tool that helps identify an appropriate configuration) would be good places to kick off.
This is a good idea. Dunlop is more dense than Talalay and, of course, has some different properties in comparison to memory foam. If you absolutely hate it, you’ll probably want to avoid 3" of medium Dunlop as your new comfort layer. If you like it, but find it slightly too firm, then the medium-soft you’re considering could be a good direction. Either way, this is a useful data point to have for your semi-DIY toolbox.
I hope I’ve given you some useful information here. Please do ask whatever questions may arise!
NikkiTMU