You can interchange some parts on a Dell machine, but certain things are built to Dell specifications and modified as such.
If you intend selling it then a complete 'working' machine would most likely sell easier. If you intend to sell 'as parts' then eBay may be a good outlet. New Dell components can be slightly pricey, and there could be a market for certain parts.
Regarding improving, would all depend on the actual model and components being used at present. Sometimes the cost of upgrading, can be non-productive, especially if you have a new buy in mind.