1) What maintenance is req'd other than the typical engine oil change, gearbox oil change, coolant flush, chain tension, plugs, etc.?
That is about it. They are pretty reliable. They have timing chains instead of belts which is a big plus.

2) What are the most common problems that plaque owners?
must common problem is probably that same with any older Italian car...oil leaks. Another is the clutch slave cylinder which is a very easy fix. I would also suggest putting in an MDS ignition. Change the motor mounts. Jalpa.ch lists things they should be changed to make the car more reliable.

3) Are most of the electric comps Bosch sourced units or other... starter, alternator, ign system, etc.? Other electrical gremlins to be concerned with... fuse boxes, poor grounds, etc.
Your best bet is to take the one out of the car and have it rebuilt by a local shop. I had my alternator rebuilt and output increased for $150. As stated above I would get the MSD ignition. Only problem with that is the tach is hit or miss if it is going to work afterwards. I am having my starter rebuilt this winter. The fuse boxes are the usual pain but as long as you keep it clean it will be good. I suggest taking out the fuses and sanding down or cleaning all the contacts. The ground straps are pretty good but, like any car you never know what you are getting until you start going through it.

4) Is there any advantage to buying a late MY year car over an earlier MY. More specifically, were there any significant non-cosmetic changes that were made over the series run?
I would usually say the newest is better because they improve things as they progress. But remember Lamborghini wasn't in the best financial situation while these cars where being built. With that said I would still buy a newer model with the 3.5L. Engine displacement was the biggest change mechanically.

5) What are the best sources for lambo parts... NOT talking about routine items that can be easily found through alternate means such as rotor/cap, spark plugs, oil /air filters.
This depends on what you are planning. You can still get some parts from the dealer. However, I would suggest having the parts custom made at a shop or using cross reference parts. In my experience you get a better part having a machine shop fabricate the part and the price is usually the same if not cheaper.

I hope this somewhat answers your questions. I will never regret buying the car. They are great cars as long as you keep them up a little bit and don't beat the crap out of them.