Hi dredge000,
I would suggest reading post #15 here which talks about the different approaches to buying or “building” a mattress. I would also recommend you work with a manufacturer who knows what could otherwise take months of research and testing to learn. In other words … finding an expert that already knows what you would need to learn and sells high quality and value mattresses that can be customized and layered the way you want them is usually more effective than becoming an expert yourself.
If you have done enough research and local testing on many different layering combinations with known specs to know within a small margin of error exactly what layers and components that you need … and if the outcome of the project is less important than the experience or just the “fun” of learning how to build “your perfect mattress” with all the mistakes and ordering of “replacement” layers that they may involve … it can be a great idea. Without this personal experience and research or unless you are one of the fortunate few that can sleep on anything … it can be very “risky” and more expensive both in terms of cost and in terms of quality of sleep than you may suspect. It would be rare that the savings alone of this approach would be worth the risk IMO.
If you choose to go in this direction anyway … the first thing I would suggest is to reconsider using Foam by Mail for any of your layers which IMO would be a big mistake for any purchase where the real specs of what you receive is important. Post #2 here along with post #2 here has more about this. If you order from an outlet that doesn’t describe their products accurately and receive a layer that is different from what you think you ordered … you will have no way to know what changes to make if the layers you order don’t work the way you expect and changes are likely if you aren’t certain about what you need. This can become a very costly exercise because you would have no accurate reference point about about what you are actually sleeping on. There are some alternative and more reliable sources listed in post #4 here.
The next thing I would suggest is enough research and local testing to know your own personal needs and preferences in a mattress rather than the “theory” of what layers may work for you only to discover that you may not fall within the “averages” that most guidelines (such as those in the mattresses section of this site and elsewhere) are based on. Each of us can be more “unique” than we may suspect in our needs and preferences in a mattress
I think that anyone can be successful going in this direction if they have the knowledge to recognize dubious materials or outlets, if they have the time, patience, and money to go through the learning curve and the expenses it may involve, and if the “spirit” of the project is more about learning and having fun than about a specific outcome on the first few tries. For example … is your choice of the Invigo based on its price or is this specific combination of “unknown” polyfoam and “unknown” latex part of an overall design?
If this is only about saving money … it may well lead to paying a price in the quality of your sleep and in actual cost until you have gone through enough trial and error to get it right (unless you can sleep on anything of course). I would also ask if it’s worth the time you would need to spend in research and local testing and the high risk of making mistakes that would lead to ordering more layers and could end up costing more than an approach where you worked with someone who already knows what you would need to learn to make this approach effective. In other words are any potential cost savings worth the time and research involved and the risk that you may not end up with a suitable mattress until you have spent more than you would have with a local purchase or working with an online manufacturer.
If this is about the challenge of building your own mattress and you are OK with the possibility that it could end up costing you more than an alternative approach and that the time and effort involved to lessen the risk is worth it … then it could be a great idea 
Phoenix