Hi Nemix,
If you have carefully tested the mattress/topper combination in person and you are confident that it would be a good match for you in terms of PPP and that you would sleep well on it then it would have the same risk of buying a mattress without a topper and would certainly be well worth considering. I would just make sure that it’s not “on the edge” of being too soft for you so that a relatively small amount of foam softening in either the mattress or the topper can take you outside the comfort/support range that would be suitable for you (see post #2 here).
On the other hand if you aren’t confident that your testing would be a good indicator of how well you will sleep on the mattress or if you are concerned about what will happen during the break in and adjustment period then I would buy the mattress first and sleep on it for a few weeks (if possible) so that you can go through some or all of the break in and adjustment process and then based on your experience you can decide on whether you need a topper at all and if you do then in combination with the topper guidelines I linked you can use your actual experience to decide on the type, thickness, and firmness for a topper that would have the least possible risk and the best chance of success.
Phoenix