What type of sander could be used for both sanding spackle and doors?
While I would be lost without a belt sander and a detail sander, for most medium to fine finishing work, I would vote for a variable speed, 6" random orbit sander, with velcro pad (as opposed to PSA (pressure sensitive adhesive).  

Get one with 6 hole dust pickup for integration with a shop vac, such as..


What are the relative ad/disadvantages of random orbit sanders (some might ask)?


The 6 inch size is more powerful than the 5s and covers more area (and bridges a wider area (important for flattening walls, rather than following the the contours)). 
The 6 hole sanding disks allow good dust pickup when suction is provided. I frequently don't need a dustmask for smaller jobs. 
The random orbit allows for a smoother scratch pattern. Be sure to work through the grits to eliminate the sanding marks of the previous grits. For Drywall sanding I would use 80-120-150. For wood I would use one more 80-120-150-220 and maybe 50 for really rough surfaces
Also, the hook-and-loop sanding discs can be reused if desired (and not worn out).
Disadvantages are: large size for detail work. Possibly too aggressive for very light work (wood block and a sheet of fine sanding paper would be better, in that case). Heavy for overhead work.