I have a T480 and the battery will last me 2 days on a charge for my typical use. Since it has two batteries, I can swap the external one without having to plug in or shut down. There are lots of parts available and you can find used or refurbished laptops at a reasonable price.
The downside with the T480 is a lack of PCIe lanes. The thunderbolt only has 2 lanes, which is not so good for an external GPU. The NVMe SSD is also only 2 lanes, but I still get around 1.5GB/s, which is plenty fast for me.