This Festool sander appears to be a dual purpose machine in that it has 2 operating heads - triangular and circular. I've no doubt that it is a top quality unit, as with all Festool, but it comes at a price. The actual finish you can get depends more on the operator and the abrasives used than the machine.
Personally when making anything with an internal finished surface I like to get the best possible sanded surface before assembly. I then attempt to preserve that finish with protection through the gluing process. Inevitably there are occasions when, despite my best efforts, there is a need to tidy up before applying the finish product. This I do either by hand or use of a triangular style detail sander.
If you want to go for a detail sander I would recommend a multitool such as the Bosch Multitool.
My library of sanders these days is (in order of usage)
Bosch Multitool,
Metabo ROS,
DeWalt Palm,
Elu Belt ,
Hitachi 1/2 Sheet. All do an excellent job. The Hitachi has really been replaced by the Metabo which is easier to live with.
I usually finish sanding at 150 for oil finishes and 240 for lacquers/varnishes, denibbing with 320 between coats.
Brian