The Taig stuff is solid and a great value. The lathe is small, not much bigger than a watchmakers lathe. It is good for what it is, but don't expect to do any kind of heavy work with it. It is a small precision tool, and for heavy stuff you would be better off with something else. On the other hand the mill is a sturdy little beast and will hold up to pretty much whatever you can throw at it. For benchtop machines it is a winner.
As for servos, they don't do as well for this size machine. Steppers are not only just as good, but actually arguably a lot better for smaller machines. Servos are a requirement for bigger machines, but the math is a lot different here. I wouldn't bother with the extra expense and trouble.
The Taig ways are indeed aluminum with a hard coat, and they are that way for good reason. You shouldn't worry about durability, they will hold up about forever, you just have some restrictions on the coolant you can use is about all.
Ballscrews can be added, but it isn't really worth it to do to a Taig. If you need something with ballscrews, its easier and cheaper just to start with a machine that has them. For accuracy, ballscrews are overrated in benchtop machines, especially as cheap rolled ballscrews and bearing blocks are usually less accurate than good screws. They still have advantages for longevity in production environments, but most of their real advantages just disappear at this scale.
A Taig isn't a $15k machine, which is what machines this size cost with steel ways and ground ballscrews and the rest. It was designed to give very close performance to those machines, and for under a grand, which it does astoundingly well. If you really need a $15k machine though, its better to just start out with one than try to monkey a Taig into the job.