Thanks all for the helpful comments - it seems like the Thinkpad T420/T450 seem to be the preferred choice although I am really happy to see that there are also

Thanks all for the helpful comments - it seems like the Thinkpad T420/T450 seem to be the preferred choice although I am really happy to see that there are also quite a few other options. System76 also sounds great because they specialize in Ubuntu and I've worked on dell laptops before and love their build quality so it's going to be a tough choice :)