SSDs are wasted if you don't use them as the main boot drive. All the performance benefits are far more useful with your OS and Applications running from it. I mean, would you want to access your data files quickly but not get the benefits of system responsiveness and quicker booting? It's a huge difference too, combined with the latest generation of CPUs, you're likely to really feel the change. In fact, HDD technology really hasn't changed for coming close to 20 years. SSDs came about precisely because spinning disks were causing the main bottleneck in most computers.
A word of caution, when an SSD dies, it's like it's been switched off, no warning, no chance to recover data, I've had this happen with one of the early ones. However, they are much better these days, frankly it should outlast your new laptop's useful life. Another benefit with a laptop to having one, you can freely move around with the laptop, without risking damage to the HDD, since there are no moving parts. As such, if you don't need a huge amount of space, a 256GB or 512GB drive may be all you need. If not though and you are looking for more built-in storage, firstly avoid so called SSHDs, or Hybrid Drives which have a small SSD cache and a normal HDD. If either part dies, the whole does. They aren't really cheap enough and are nothing like as efficient, it's a half-baked solution IMO. Getting a laptop with space for two drives may not be a huge issue, since you aren't looking for a budget model, you should be able to achieve that. Just bear in mind, that puts pay to the moving around with it safely thing I mentioned before. I have an XP laptop with a 60GB drive that's just fine for my needs, but for a more modern OS, you'll be needing 256 realistically. I'm already courting the idea of upgrading mine to 512 (Desktop), and chucking the 256 into another laptop I have.
Indeed Appalachian and all such HD textures must be used with DirectX hardware rendering or they won't work. I wish I could tell you absolutely 100% this will happen, but the reality is that there is no real way to know. Unless by some miracle you can test-drive a machine and install SC4 on it as part of that. But the Intel drivers as far as I know will work, but people do get issues with some newer GPUs. If you have the option to buy a machine you can return with no fuss for a brief period, that would give you a little time to ensure it will play SC4 properly or get your money back.