Hi Imyyap,
You can see my comments about choosing a firmer mattress first with the intention of adding a topper later in post #2 here. In most cases I would avoid this approach because of the uncertainty involved with two purchase choices instead of only one and choosing a topper that would be suitable in terms of PPP for a specific person on a specific mattress can sometimes be almost as difficult as choosing a mattress that doesn’t need a topper in the first place. I would generally focus on choosing a mattress that is likely to be a suitable match without a topper (unless you can test the combination in person or you are purchasing both online as a “set” that is designed to work together and they both have a good return/exchange policy) and then use the option to add a topper as a “backup” strategy in case your initial choice is too firm and doesn’t turn out as well as you hoped for rather than a “primary” strategy.
It would also be very difficult to avoid buying a mattress in your budget range that doesn’t have lower quality and less durable materials in the comfort layer that would be a weak link in terms of the durability and useful life of the mattress/topper combination.
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You would probably be better off using your budget to choose a mattress that doesn’t have any lower quality materials or weak links and that doesn’t need a topper to be a good “match” for you in terms of comfort, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences).
Phoenix