Hi shangkan,
One of the challenges of buying a firm mattress and then adding a topper when you can’t test the mattress/topper combination in person is that choosing a suitable topper that is a good combination for you and the mattress you are using it on (different mattresses will affect the choice of topper that works best for a particular person) that can be almost as challenging as choosing a mattress that doesn’t need a topper in the first place because the only way to know whether the combination will be a good “match” for you will be based on your own personal experience (see post #2 here).
It can also be more costly than just buying a mattress which includes the same materials as the topper as a layer inside the mattress itself and you would be avoiding the risk of having additional materials in your base mattress that you may not need or that may be lower quality foam (that can soften or break down more quickly) or that may be softer than what would be ideal for a transition layer under a softer topper. At the very least I would make sure that the topper has a good exchange or return policy so there would be less risk of buying a topper that doesn’t turn out as well as you hoped for in combination with your mattress. I would also make sure that you can find out all the specifics of the materials inside the base mattress so you can make sure it meets the quality/durability guidelines here and that there are no weak links in the mattress.
Having said that … if you do choose a suitable mattress/topper combination which turns out to be a good “match” for you in terms of PPP (Posture and alignment, Pressure relief, and your Personal preferences) then it does have the advantage of being able to replace just the topper without replacing the entire mattress if it softens or breaks down before the upper foam layers in the mattress (which is likely because a sleeping system will tend to soften or break down from the top layers down) or if your needs or preferences change over time and a topper can also help extend the useful life of a mattress underneath it as well.
The better options or possibilities I’m aware of in and around the Los Angeles region which would include Irvine (subject to making sure that any mattress you are considering meets that quality/value guidelines here) are listed in post #2 here.
If you are attracted to the idea of designing and building your own DIY mattress out of separate components and a separate cover then the first place I would start is by reading option 3 in post #15 here and the posts it links to (and option #1 and #2 as well) so that you have more realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project … the best approach to a DIY mattress is a “spirit of adventure” where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen).
If you decide to take on the challenge then I would either use the specs (if they are available) of a mattress that you have tested and confirmed is a good match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) as a reference point or blueprint and try and “match” every layer and component in your reference mattress (including any springs, any foam layers, and the cover) as closely as possible or use a “bottom up” approach (see post #2 here).
Phoenix