1) Is there any advantage to going with a later model 325/330... were there any note-worthy changes between the years of 02-05 that would make the later model a better buy?
Better to get an 02 if depreciation is an issue, you get a better deal and if the car was well maintained and is not a lemon it is bullit proof!
2) In terms of ongoing maintenance and upkeep, how expensive (or not) is it, to own one of these cars?
If you take it to the dealer you will get hosed LARGE! Better to find a BMW mechanic near you, dont go for the posh location, my mechanic is in a crappy gas station and does great work is dirt cheap.
3) In terms of little maintanance and repairs that might crop up later in the life of this car, is it something that DIY car guy like me can handle or will I have to run to the dealership, everytime something goes wrong, because of the car being too "unique" and not like any other car to work on?
You can do many of the maintenance things yourself, oil change, brake flush, changing air filters (cabin and intake), inspections, resetting the maintenance lights on the dash, etc. Again if you take the car to the dealer you are going to get hosed.
Remember even if the car is under warrantee you will still get all the bs charges that are not covered under the warrantee, and you will be forced to continue going back to the dealer to get ripped off again and again.
Good luck to you!