As for pointers on building, there are some general tips included with the plans. These include things to watch like using braces and insulation and being careful to observe polarity when connecting the wires. But there are a lot more good suggestions discussed here in the forum. You might want to look thorugh the archives by using the search function. Try specific words like "bracing" or "joints" or "insulation." There are some really talented cabinetmakers that frequent this forum and they have given some excellent tips.

About upgrades, some woofers can be substituted in a particular design and others cannot. Sometimes you can swap a woofer in a cabinet with some changes, like changing the port. But if you are considering an upgrade path, this is something you might want to think about ahead of time.

As an example, the Stage, Premium, Audiophile and Professional Series cabinets are all the same, so the woofers are interchangeable. But the Theater Series is not. You can do some port modifications to a Theater Series seven π and make a Stage, Premium, Audiophile or Professional Series seven π-18. But that requires modification, so you might want to consider that as you make your initial decision.